"Invalid Printer specified" error can be resolved by reconfiguring printer settings, modifying the printer assignment, printing to the server with redirect/remote printers, and adding another page to the ACCTHandheld web service.
Utilizing a printer from a mobile device is a complicated setup process. In the event you have issues with some users printing reports from within Acctivate Mobile, you may consider reviewing the proper setup of Mobile Printing here.
A common scenario in which reports don't print and return an "Invalid Printer specified" error from Mobile is that the printer assignment needs to be set as the web service would see it, but the only way to set up the printer assignment is through Acctivate desktop, which sets it up as the way the currently logged-in user sees it. Basically, the only time the two are the same is if a printer is directly connected to the web server, and the user sets the printer assignments while logged into the web server. But there are some tricks even with that:
A company may have a network printer (i.e., one that has an ethernet port and plugs directly into the network), but they don't actually have the printer installed and configured on the server. They do, however, have it configured on the workstations. They remote desktop to the server, and by default, your workstation printers are available as redirects on the server, so you can print from the server and it'll print on your local printer (but since it's a network printer, people don't think of it as being "local"... it's still using the print driver on your local machine though). If they run Acctivate over that remote desktop connection and do the printer assignments from there, it'll set it to the redirected printer, which disappears as soon as they log off. And when they try to print from ACCT mobile, it gives an "Invaild printer specified" error.
A solution for this is to add another page to the ACCTHandheld web service where the user can configure the printer assignments for mobile. That way, it can show the list of printers that are available to the web service, and perhaps do some extra checks to make sure it's not a RDP-redirected printer.