This is a bug introduced by #3008. Default config (AllowProxyAuthType=Off) should be ok if web is using same authtype as users. It is backward-compatibility issue.
AllowProxyAuthType=Off
Metadata Update from @jcupova: - Custom field Size adjusted to None - Issue tagged with: testing-ready
PR 3164
Metadata Update from @jobrauer: - Issue tagged with: testing-done
Commit e23e7d6c fixes this issue
Commit 969f1025 fixes this issue
This issue has been migrated to Fedora Forge: https://forge.fedoraproject.org/koji/koji/issues/3163
Please continue any further discussion there.