This can occur when the thum.mdb file has been moved from the default location of c:\program files\data to a network share.
Version 2.0.x of the THUM software has a portion that runs as a windows service so that readings will continue to be taken even when no user is logged into the computer. By default, services run as the local system account. The local system account does Not have any network access. If you want to save data to a network location you need to change the user account the service runs under. It will need to be an account that has local admin permissions as well as write permissions to the network location of your database. You can change the service account user under the Windows Services manager.