diff --git a/bundles/org.openhab.core.automation.rest/src/main/java/org/openhab/core/automation/rest/internal/ThingActionsResource.java b/bundles/org.openhab.core.automation.rest/src/main/java/org/openhab/core/automation/rest/internal/ThingActionsResource.java
index e09887906..0a1dc86fa 100644
--- a/bundles/org.openhab.core.automation.rest/src/main/java/org/openhab/core/automation/rest/internal/ThingActionsResource.java
+++ b/bundles/org.openhab.core.automation.rest/src/main/java/org/openhab/core/automation/rest/internal/ThingActionsResource.java
@@ -131,7 +131,8 @@ public class ThingActionsResource implements RESTResource {
             if (actionUIDs.isEmpty()) {
                 return;
             }
-            thingActionsMap.computeIfAbsent(thingUID, thingUid -> new ConcurrentHashMap<>()).put(scope, actionUIDs);
+            Objects.requireNonNull(thingActionsMap.computeIfAbsent(thingUID, thingUid -> new ConcurrentHashMap<>()))
+                    .put(scope, actionUIDs);
         }
     }
 
@@ -165,7 +166,7 @@ public class ThingActionsResource implements RESTResource {
     @Produces(MediaType.APPLICATION_JSON)
     @Operation(operationId = "getAvailableActionsForThing", summary = "Get all available actions for provided thing UID", responses = {
             @ApiResponse(responseCode = "200", description = "OK", content = @Content(array = @ArraySchema(schema = @Schema(implementation = ThingActionDTO.class), uniqueItems = true))),
-            @ApiResponse(responseCode = "404", description = "No actions found.") })
+            @ApiResponse(responseCode = "204", description = "No actions found") })
     public Response getActions(@PathParam("thingUID") @Parameter(description = "thingUID") String thingUID,
             @HeaderParam(HttpHeaders.ACCEPT_LANGUAGE) @Parameter(description = "language") @Nullable String language) {
         Locale locale = localeService.getLocale(language);
@@ -174,7 +175,7 @@ public class ThingActionsResource implements RESTResource {
         List<ThingActionDTO> actions = new ArrayList<>();
         Map<String, List<String>> thingActionsMap = this.thingActionsMap.get(aThingUID);
         if (thingActionsMap == null) {
-            return Response.status(Response.Status.NOT_FOUND).build();
+            return Response.noContent().build();
         }
 
         // inspect ThingActions
diff --git a/bundles/org.openhab.core.io.rest.core/src/main/java/org/openhab/core/io/rest/core/config/ConfigurationService.java b/bundles/org.openhab.core.io.rest.core/src/main/java/org/openhab/core/io/rest/core/config/ConfigurationService.java
index c0163110a..cbd1b849d 100644
--- a/bundles/org.openhab.core.io.rest.core/src/main/java/org/openhab/core/io/rest/core/config/ConfigurationService.java
+++ b/bundles/org.openhab.core.io.rest.core/src/main/java/org/openhab/core/io/rest/core/config/ConfigurationService.java
@@ -155,8 +155,11 @@ public class ConfigurationService {
      * @return old config or null if no old config existed
      * @throws IOException if configuration can not be removed
      */
-    public Configuration delete(String configId) throws IOException {
+    public @Nullable Configuration delete(String configId) throws IOException {
         org.osgi.service.cm.Configuration serviceConfiguration = configurationAdmin.getConfiguration(configId, null);
+        if (serviceConfiguration == null) {
+            return null;
+        }
         Configuration oldConfiguration = toConfiguration(serviceConfiguration.getProperties());
         serviceConfiguration.delete();
         return oldConfiguration;
diff --git a/bundles/org.openhab.core.io.rest.core/src/main/java/org/openhab/core/io/rest/core/internal/channel/ChannelTypeResource.java b/bundles/org.openhab.core.io.rest.core/src/main/java/org/openhab/core/io/rest/core/internal/channel/ChannelTypeResource.java
index 8d40c006c..c51c1ea71 100644
--- a/bundles/org.openhab.core.io.rest.core/src/main/java/org/openhab/core/io/rest/core/internal/channel/ChannelTypeResource.java
+++ b/bundles/org.openhab.core.io.rest.core/src/main/java/org/openhab/core/io/rest/core/internal/channel/ChannelTypeResource.java
@@ -140,8 +140,9 @@ public class ChannelTypeResource implements RESTResource {
     @Path("/{channelTypeUID}")
     @Produces(MediaType.APPLICATION_JSON)
     @Operation(operationId = "getChannelTypeByUID", summary = "Gets channel type by UID.", responses = {
-            @ApiResponse(responseCode = "200", description = "Channel type with provided channelTypeUID does not exist.", content = @Content(schema = @Schema(implementation = ChannelTypeDTO.class))),
-            @ApiResponse(responseCode = "404", description = "No content") })
+            @ApiResponse(responseCode = "200", description = "OK", content = @Content(schema = @Schema(implementation = ChannelTypeDTO.class))),
+            @ApiResponse(responseCode = "400", description = "Bad request"),
+            @ApiResponse(responseCode = "404", description = "Channel type with provided channelTypeUID does not exist.") })
     public Response getByUID(
             @PathParam("channelTypeUID") @Parameter(description = "channelTypeUID") String channelTypeUID,
             @HeaderParam(HttpHeaders.ACCEPT_LANGUAGE) @Parameter(description = "language") @Nullable String language) {
@@ -150,7 +151,7 @@ public class ChannelTypeResource implements RESTResource {
         if (channelType != null) {
             return Response.ok(convertToChannelTypeDTO(channelType, locale)).build();
         } else {
-            return Response.noContent().build();
+            return Response.status(Status.NOT_FOUND).build();
         }
     }
 
diff --git a/bundles/org.openhab.core.io.rest.core/src/main/java/org/openhab/core/io/rest/core/internal/service/ConfigurableServiceResource.java b/bundles/org.openhab.core.io.rest.core/src/main/java/org/openhab/core/io/rest/core/internal/service/ConfigurableServiceResource.java
index cf27785a4..496ae00c8 100644
--- a/bundles/org.openhab.core.io.rest.core/src/main/java/org/openhab/core/io/rest/core/internal/service/ConfigurableServiceResource.java
+++ b/bundles/org.openhab.core.io.rest.core/src/main/java/org/openhab/core/io/rest/core/internal/service/ConfigurableServiceResource.java
@@ -150,7 +150,7 @@ public class ConfigurableServiceResource implements RESTResource {
         if (configurableService != null) {
             return Response.ok(configurableService).build();
         } else {
-            return Response.status(404).build();
+            return Response.status(Status.NOT_FOUND).build();
         }
     }
 
@@ -275,8 +275,7 @@ public class ConfigurableServiceResource implements RESTResource {
     public Response deleteConfiguration(
             @PathParam("serviceId") @Parameter(description = "service ID") String serviceId) {
         try {
-            Configuration oldConfiguration = configurationService.get(serviceId);
-            configurationService.delete(serviceId);
+            Configuration oldConfiguration = configurationService.delete(serviceId);
             return oldConfiguration != null ? Response.ok(oldConfiguration).build() : Response.noContent().build();
         } catch (IOException ex) {
             logger.error("Cannot delete configuration for service {}: {}", serviceId, ex.getMessage(), ex);
diff --git a/bundles/org.openhab.core.io.rest.core/src/main/java/org/openhab/core/io/rest/core/internal/thing/ThingTypeResource.java b/bundles/org.openhab.core.io.rest.core/src/main/java/org/openhab/core/io/rest/core/internal/thing/ThingTypeResource.java
index 4e382dcdf..b84d2c399 100644
--- a/bundles/org.openhab.core.io.rest.core/src/main/java/org/openhab/core/io/rest/core/internal/thing/ThingTypeResource.java
+++ b/bundles/org.openhab.core.io.rest.core/src/main/java/org/openhab/core/io/rest/core/internal/thing/ThingTypeResource.java
@@ -28,6 +28,7 @@ import javax.ws.rs.QueryParam;
 import javax.ws.rs.core.HttpHeaders;
 import javax.ws.rs.core.MediaType;
 import javax.ws.rs.core.Response;
+import javax.ws.rs.core.Response.Status;
 
 import org.eclipse.jdt.annotation.NonNullByDefault;
 import org.eclipse.jdt.annotation.Nullable;
@@ -151,8 +152,8 @@ public class ThingTypeResource implements RESTResource {
     @Path("/{thingTypeUID}")
     @Produces(MediaType.APPLICATION_JSON)
     @Operation(operationId = "getThingTypeById", summary = "Gets thing type by UID.", responses = {
-            @ApiResponse(responseCode = "200", description = "Thing type with provided thingTypeUID does not exist.", content = @Content(schema = @Schema(implementation = ThingTypeDTO.class))),
-            @ApiResponse(responseCode = "404", description = "No content") })
+            @ApiResponse(responseCode = "200", description = "OK", content = @Content(schema = @Schema(implementation = ThingTypeDTO.class))),
+            @ApiResponse(responseCode = "404", description = "Thing type not found.") })
     public Response getByUID(@PathParam("thingTypeUID") @Parameter(description = "thingTypeUID") String thingTypeUID,
             @HeaderParam(HttpHeaders.ACCEPT_LANGUAGE) @Parameter(description = "language") @Nullable String language) {
         Locale locale = localeService.getLocale(language);
@@ -160,7 +161,7 @@ public class ThingTypeResource implements RESTResource {
         if (thingType != null) {
             return Response.ok(convertToThingTypeDTO(thingType, locale)).build();
         } else {
-            return Response.noContent().build();
+            return Response.status(Status.NOT_FOUND).build();
         }
     }