diff --git a/full/src/main/java/de/ids_mannheim/korap/oauth2/oltu/service/OltuAuthorizationService.java b/full/src/main/java/de/ids_mannheim/korap/oauth2/oltu/service/OltuAuthorizationService.java
index 0ac3da6..5898beb 100644
--- a/full/src/main/java/de/ids_mannheim/korap/oauth2/oltu/service/OltuAuthorizationService.java
+++ b/full/src/main/java/de/ids_mannheim/korap/oauth2/oltu/service/OltuAuthorizationService.java
@@ -143,7 +143,8 @@
         int statusCode = e.getStatusCode();
         if (!clientId.isEmpty()
                 && statusCode != StatusCodes.CLIENT_NOT_FOUND
-                && statusCode != StatusCodes.AUTHORIZATION_FAILED) {
+                && statusCode != StatusCodes.AUTHORIZATION_FAILED
+                && statusCode != StatusCodes.INVALID_REDIRECT_URI) {
             String registeredUri = null;
             try {
                 OAuth2Client client = clientService.retrieveClient(clientId);
diff --git a/full/src/main/java/de/ids_mannheim/korap/oauth2/service/OAuth2AuthorizationService.java b/full/src/main/java/de/ids_mannheim/korap/oauth2/service/OAuth2AuthorizationService.java
index 69713f8..581fa09 100644
--- a/full/src/main/java/de/ids_mannheim/korap/oauth2/service/OAuth2AuthorizationService.java
+++ b/full/src/main/java/de/ids_mannheim/korap/oauth2/service/OAuth2AuthorizationService.java
@@ -4,6 +4,7 @@
 import java.time.ZonedDateTime;
 import java.util.Set;
 
+import org.apache.commons.validator.routines.UrlValidator;
 import org.apache.logging.log4j.LogManager;
 import org.apache.logging.log4j.Logger;
 import org.springframework.beans.factory.annotation.Autowired;
@@ -38,6 +39,8 @@
     protected OAuth2ScopeServiceImpl scopeService;
     @Autowired
     private AuthorizationDao authorizationDao;
+    @Autowired
+    private UrlValidator redirectURIValidator;
 
     @Autowired
     protected FullConfiguration config;
@@ -110,24 +113,24 @@
             throws KustvaktException {
 
         String registeredUri = client.getRedirectURI();
+        
         if (redirectUri != null && !redirectUri.isEmpty()) {
             // check if the redirect URI the same as that in DB
-            if (registeredUri != null && !registeredUri.isEmpty()
-                    && !redirectUri.equals(registeredUri)) {
+            if (!redirectURIValidator.isValid(redirectUri) ||
+                    (registeredUri != null && !registeredUri.isEmpty()
+                    && !redirectUri.equals(registeredUri))) {
                 throw new KustvaktException(StatusCodes.INVALID_REDIRECT_URI,
                         "Invalid redirect URI", OAuth2Error.INVALID_REQUEST);
             }
         }
-        else {
-            // redirect_uri is not required in client registration!
-            if (registeredUri != null && !registeredUri.isEmpty()) {
+        // redirect_uri is not required in client registration
+        else if (registeredUri != null && !registeredUri.isEmpty()) {
                 redirectUri = registeredUri;
-            }
-            else {
-                throw new KustvaktException(StatusCodes.MISSING_REDIRECT_URI,
-                        "Redirect URI is required",
-                        OAuth2Error.INVALID_REQUEST);
-            }
+        }
+        else {
+            throw new KustvaktException(StatusCodes.MISSING_REDIRECT_URI,
+                    "Redirect URI is required",
+                    OAuth2Error.INVALID_REQUEST);
         }
 
         return redirectUri;
diff --git a/full/src/main/java/de/ids_mannheim/korap/oauth2/service/OAuth2ClientService.java b/full/src/main/java/de/ids_mannheim/korap/oauth2/service/OAuth2ClientService.java
index 45dbd6b..00f9e70 100644
--- a/full/src/main/java/de/ids_mannheim/korap/oauth2/service/OAuth2ClientService.java
+++ b/full/src/main/java/de/ids_mannheim/korap/oauth2/service/OAuth2ClientService.java
@@ -98,15 +98,15 @@
         if (url != null && !url.isEmpty()) {
             if (!urlValidator.isValid(url)) {
                 throw new KustvaktException(StatusCodes.INVALID_ARGUMENT,
-                        url + " is invalid.", OAuth2Error.INVALID_REQUEST);
+                        "Invalid URL", OAuth2Error.INVALID_REQUEST);
             }
         }
 
         String redirectURI = clientJson.getRedirectURI();
         if (redirectURI != null && !redirectURI.isEmpty()
                 && !redirectURIValidator.isValid(redirectURI)) {
-            throw new KustvaktException(StatusCodes.INVALID_ARGUMENT,
-                    redirectURI + " is invalid.", OAuth2Error.INVALID_REQUEST);
+            throw new KustvaktException(StatusCodes.INVALID_REDIRECT_URI,
+                   "Invalid redirect URI", OAuth2Error.INVALID_REQUEST);
         }
 
         // boolean isNative = isNativeClient(url, redirectURI);