Hello,
This issue appears to be similar in nature to a related issue with the unofficial 3rd party fork of Bitwarden known as Vaultwarden.
Can you check to see if your self-hosted service is running Bitwarden or Vaultwarden?
If it is the latter, there is currently an open issue regarding this and most likely will be fixed in a later release by their devs permitting.
Appears to be caused by a mishandling of the backend 3rd party server as noted:
BlackDex
Confirmed. It looks like it is trying to access the token,[INFO] (login) POST /identity/connect/token, but not allowed.
Cheers,