@Akram-Khalil
What do you see logged in the nifi-app.log when you attempt to access the NiFI UI?
I don't think this is related to your ldap configuration, but I don't have your authorizers.xml or nifi.properties to verify your configuration setup.
This exception is more related to authorization and not authentication.
It is more likely related to missing "proxy user requests" authorization being granted to the NiFi node certificates.
But this should be easy to resolve if enough information can be shared, which includes nifi-userlog output which will show the user client identity being denied authorization and the above mentioned configuration files.
You can also file a Cloudera support ticket if you have a Cloudera support contract and this can be solved live over a call.
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Thank you,
Matt