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.
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Comment by @Hudeler
https://example.com/works as expected buthttps://idp.example.com/causes theINVALID_FORM_BODY/INVALID_URL_FORMATerror and the config does not persist.