In addition to checking the Options Manager settings, I just wanted to clarify why you're seeing one aspect of LDAP working while others don't.
Service Desk has one or two daemons (depending on the version you are using) that perform user authentication, boplgin and bopauth_nxd. When a user attempts to login it will check the access type associated to the user, and if it's configured for "Windows Authentication" the daemons will first attempt to find the contact record on the local server OS, after that attempt it will check for users in the domain that the server is joined to.
This whole process is completely outside and independent of the daemon that performs the ldap imports or merges, so the fact that authentication works but the imports failing is a possibility.
The LDAP options manager settings are ONLY used for the LDAP import/merge processes, they have no effect on the user's ability to authenticate.