Accessing the VOB requires navigating the structure of directory elements, which is stored in the VOB database. The db_server process is retrieving this. All database access by the server processes, both read and write access, has to be co-ordinated though lockmgr, so if lockmgr is failing, you have no access. If you still have the problem, and require further assistance, please raise a support case.
Traversing the VOB in a view context generally uses the vobrpc server process. Almost every other action will cause a db server process to be invoked. I would expect to see errors in both logs if users cannot traverse directories. The users should see "access denied" or other errors trying to CD into the VOB if the vobrpc_server processes are seeing the same issue.