I have almost the same problem...
I upgraded OSTicket from version 1.15.2 to version 1.17.2, going from PHP7.3 to PHP8.1 and MySQL/MariaDB version is 10.3.36. My server is a Debian 10 Virtual Machine.
I had previously upgraded my plugins including the LDAP plugin to 0.6.2 (Is it really the lastest version ??) and added a LDAP Search User.
The upgrade is done without problems, the integration of the PHP8.1 Extensions made by myself appears correct after the upgrade (Dashboard > Information looks great with valid sign !).
No errors in the Apache2 logs for example.
I upgraded as a Local (root) account so when I try to reconnect with my AD account I get an "Access denied" message.
I searched the forum and found a solution that works for agents here: https://forum.osticket.com/d/101553-ldap-not-enter-whit-user/66
You have to change the authentication method to "Local", save and change again to "LDAP" and save. After that the agent account can be reconnected by the LDAP method
However my "Standard" users who are not "Agents" still can't connect on their side, they get an "Access denied" message.
I have read many topics in the forum and I can't find any solution, even if I try to change in the database directly in the ost_user_account table the "backend" column by "NULL" and then by "ldap.client" it still doesn't work.
I'm pretty sure it's not a PHP version problem for my installation (between 8.0 or 8.1) because I don't have an error like you and according to the last post from @KevinTheJedi .
However, be aware that you have to completely deactivate/uninstall your old versions of PHP to avoid any problems of extension conflicts between versions.
I'm still looking for a solution but if you have an idea to help me, I'm interested!