I have configured SABnzbd the same as my *arr apps.
When I go to my sab URL, I am redirected to the Authentik prompt as expected. However after entering my credentials, I am then sent to the SABnzbd login prompt.
For the *arr apps, the fix was to modify config.xml to set to External. But SABnzbd doesn't appear to have a similar option.
How have others resolved this issue?