Index: oidc-authenticator/src/main/java/org/xwiki/contrib/oidc/auth/internal/OIDCClientConfiguration.java
IDEA additional info:
Subsystem: com.intellij.openapi.diff.impl.patch.CharsetEP
<+>UTF-8
===================================================================
--- oidc-authenticator/src/main/java/org/xwiki/contrib/oidc/auth/internal/OIDCClientConfiguration.java	(revision 0dad1d554088428dc6bf1622b092b2cf8da3c2a9)
+++ oidc-authenticator/src/main/java/org/xwiki/contrib/oidc/auth/internal/OIDCClientConfiguration.java	(revision 9fd44cfb0e3e5b903cb8721d76d3cb01413f7dad)
@@ -138,6 +138,8 @@
 
     public static final String PROP_ENDPOINT_USERINFO = PROPPREFIX_ENDPOINT + UserInfoOIDCEndpoint.HINT;
 
+    public static final String PROP_ENDPOINT_ACCEPTHEADER = PROPPREFIX_ENDPOINT + "acceptheader";
+
     /**
      * @since 1.21
      */
@@ -429,6 +431,14 @@
         return getEndPoint(LogoutOIDCEndpoint.HINT);
     }
 
+    /**
+     * @since 1.21.4
+     */
+    public String getAcceptheader()
+    {
+        return getProperty(PROP_ENDPOINT_ACCEPTHEADER, String.class);
+    }
+
     public ClientID getClientID()
     {
         String clientId = getProperty(PROP_CLIENTID, String.class);
Index: oidc-authenticator/src/main/java/org/xwiki/contrib/oidc/auth/internal/OIDCUserManager.java
IDEA additional info:
Subsystem: com.intellij.openapi.diff.impl.patch.CharsetEP
<+>UTF-8
===================================================================
--- oidc-authenticator/src/main/java/org/xwiki/contrib/oidc/auth/internal/OIDCUserManager.java	(revision 0dad1d554088428dc6bf1622b092b2cf8da3c2a9)
+++ oidc-authenticator/src/main/java/org/xwiki/contrib/oidc/auth/internal/OIDCUserManager.java	(revision 9fd44cfb0e3e5b903cb8721d76d3cb01413f7dad)
@@ -183,6 +183,12 @@
         userinfoHTTP.setHeader("User-Agent", this.getClass().getPackage().getImplementationTitle() + '/'
             + this.getClass().getPackage().getImplementationVersion());
         this.logger.debug("OIDC user info request ({}?{})", userinfoHTTP.getURL(), userinfoHTTP.getQuery());
+
+        // explicitly set Accept header if specified
+        if (StringUtils.isEmpty(this.configuration.getAcceptheader())) {
+            userinfoHTTP.setAccept(this.configuration.getAcceptheader());
+        }
+
         HTTPResponse httpResponse = userinfoHTTP.send();
         this.logger.debug("OIDF user info response ({})", httpResponse.getContent());
         UserInfoResponse userinfoResponse = UserInfoResponse.parse(httpResponse);