It looks like the database has been corrupted or indeed reached its capacity.
You can try to open it (MDB file in the application's folder) in MS Access, deleting some old records and then choose Repair & Compact Database from the Tools menu in MS Access.
If the above does not help, I am afraid the database can be lost for good as the 'unspecified error' is returned from the built-in Windows database engine. In this case you can uninstall Print Inspector completely and then install version 7 from scratch. It uses a new database engine called SQLite which offers a greater capacity and performance.