An exception happens if VPOP3 encounters a problem it did not expect. In some cases it can recover automatically, but in other cases that isn't possible.
In VPOP3 2.6 and earlier it was quite common for there to be database corruption problems with the SQLite database which was used in those versions.
These can be identified because if you look in the exception error message which is emailed to the administrator there is a 'Stack Dump' and some of the values in the Stack Dump will start with '673', eg '673c322a'. These numbers are memory addresses within the SQLite library component and usually indicate a problem with an SQLite database.
If you look at the exception message text, this may suggest where the problem has occurred.
If it contains the text: