Maybe the URL redirect to the OIDC login isn't allowed because of the IFrame. Have you checked different (unsecure) browsers and have you checked the browser console (F12) for messages? Maybe your CSP is incorrectly configured.
I haven't figured it out why this is still not working on our DEV environment nevertheless, we have deployed it to the test stage and here it works.
Will double check the CSP rule again, but I'm woundering, why it is working now with the same CSP rule on the test stage. Anyway, will let you now as soon I found the root cause on our DEV environment.