Please confirm with the customer that they are using a secure Process Repository. This can be completed by looking for a folder called Process RepositoryCRED where 'Process Repository' is whatever their data folder is called.
This message is in effect saying that their account is not valid to access the Process Repository.
The most likely reason for this is an expired password. This can be checked by getting them to login as that account.
If this is the case they should set the password to never expire and setup the secure Process Repository again by running the repocred utility.
If they urgently need access - rename the cred.ini file and ensure the NTFS permissions are correct on the Process Repository in order to ensure correct rights are given