Hi bsa,
If you don't use SQL Server, then fair enough, Let me at least point out that the error noted there is due to more volume over the Terminal server than can be handled by the system.
Here's something to try. Can you print invoices from within the office (ie not over the Terminal server)? Give this a try for 1, 2 or 4 users simultaneously. If you do not get the Named Pipe errors, then you can be sure your Terminal server setup cannot handle the increased volume, and some settings will have to be modified. We cannot advise what possible settings will be affected or require changes.
Also make sure that every station that chooses to print invoices has the correct printer driver installed, and that if the 'Specific Printer' option is used in File Options, then all workstations should have that driver installed as well.
Regards,