Captive portal LDAP authentication not working but access tester works

[Mistery]

Hi all, I am experiencing some issues with getting captive portal LDAP authentication working. The external LDAP authentication server was configured correctly in OPNsense and it works fine when testing via System > Access > Tester, however the captive splash screen authentication returns authentication failed.
Does anybody have any hints on what could be the root cause of this issue ?
I am running latest 21.1.5 release.