It looks like the URL validation does not like subdomains for the URL in the "Issuer" field. E.g. https://example.com/ works as expected but https://idp.example.com/ causes the INVALID_FORM_BODY/INVALID_URL_FORMAT error and the config does not persist.
Try including the application in the url. For Authentik I have https://auth.example.com/application/o/fluxer/ and everything saves but then fails silently with the 400 in the logs.
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Comment by @nadd3r
https://auth.example.com/application/o/fluxer/and everything saves but then fails silently with the400in the logs.