You do not have permission to access the instance 'CustomerName'
This has been found to occur when the DA Customer database has been manually updated in an effort to remove orphaned users and their associated permissions through the article linked below. The UPDATE query (supplied by Veritas Technical Support) applies an end date to one of the tables associated to the permission history of the impacted user. By contrast, a user with active permissions will find a NULL entry for the end date.
If feasible, the quickest solution is to restore the database to a point prior to the issue taking place. Alternatively, one can find the roles associated to the user in question, remove all of the role assignments, then reapply the role assignment. It has also been found that sometimes the Role itself needs to be reset by changing an Allow permission to a Deny permission, then back to Allow. The same needs to be done with the Deny permissions: First change to Allow then change back to Deny. This needs to be done on all permissions of the Role. Once everything has been switched back to its original state, click Save.
How to find users with orphaned Permissions in the Accelerators