Hi @ada_vlachou,
It seems like the custom domain is working, judging from your statement, the error happens during the redirection to the callback URL, specifically with the “Failed Exchange: Failed to verify code verified” error message.
You can also verify that the custom domain works by checking on your Dashboard > Branding > Custom Domains.
As for the Failed-to-verify code verifier error, I suggest referring to our Failed to verify code verifier knowledge solution.
Please let me know if you have any questions after reviewing the knowledge solution.
Thanks,
Rueben