Steps to reproduce:

1. Filling in all needed info for providing LDAP Login. 
2. Autentication -> LDAP Settings
3. when we fill in all needed info on our zabbix LTS 6.0 ( migrated from 5.2) and click on the update button  we get these errors i needed to communicate with developpers. ( see print screens )
4. for the migration i used this guide. https://bestmonitoringtools.com/upgrade-zabbix-to-the-latest-version
5. After migration all seems to work fine besides the LDAP login ( we did not implemented LDAP before migrating )
6. On the back up of the previous version of zabbix (5.2) LDAP is working out of the box. 

Result:
See screenshot...

Expected:
See screenshot....
See attached patch file...