I have a USB printer connected to my DiskStation and use it from both a MacBook and a PC. Both were able to print for a very long time.
Suddenly, both could not print any more. MacOS X showed the printer as "paused" (yellow dot) and clicking the "play" button in the printer queue view did not help. Windows said that the printer was occupied or something to that effect (not sure what the exact error message was, sorry) in the printer queue window, next to a print job.
DiskStation decided to rename the printer. Originally, its name was usbprinter and that is what had to be entered as "Queue" in both Windows and OS X. One day, it got changed to usbprinter2 - and that is what caused all the trouble. Confusingly, in the DSM setup interface, the printer's name is usbprinter2, but the field "queue name" is empty.
In OS X, remove the printer and add a new one, according to the official documentation. Only, instead of using usbprinter, as it says there, use usbprinter2.
In Windows, I set up the printer manually (not using the Synology Assistant as described here). In the printer settings, I had to add a new port, enter the IP address, let it search (to no avail) for a port and enter custom settings like this (sorry for this being in german, but it should be pretty obvious how this works):
Also, note that I entered usbprinter2 as queue name.
Worked like a charm!
Also, check if it is actually usbprinter2 in your case: In DSM Control Panel go to the printer settings (in External Devices) and check the printer's name.
The answer from David worked for me as well.
In order to open the "Configure Standard TCP/IP Port Monitor" dialog, I used "printui /s /t2" -> "Print Server Properties" dialog -> Select the underlying port and choose [Configure port ...].
