This happens when vpXprint cannot resolve the name of the printer and is usually due to the fax driver installing higher on the printer names list in Windows.  So it most likely occurs when a customer installs a new all-in-one printer/fax/scanner.  There are 2 solutions to try.
  1. First, you can try moving the printer name above the fax name in the list.
  2. If this doesn't solve the problem then rename the fax driver at the beginning of the name.  I usually just add FAX before the driver name to distinguish it.