MF4690 Stuck at Processing/Data When Print Action Requested

MF4690 will no longer print documents over LAN from any of our computers using PCL5e or PCL6 drivers (two Windows 7 machines, three Windows XP machines). UFRII LT driver is OK.

After printing from Windows, the processing/data illuminator on the printer turns green and stays lit, but the print action never completes.

For documents under 256 KB, Windows spooler does not report any errors; for larger files, it gets to 1 MB and then states "Error - Printing".

Navigating to "System Monitor" -> Print Status on the printer itself shows the correct name of the file to be printed, and the "Job Status" page of the web interface even states that it is printing.

I can, however, print a test page from all five computers just fine.

One other strange thing I noticed is that the toner gauge initially reports 20%, but after the print action fails, it reports 100%. After turning the printer off and on, it reports 20% again.

I ran a DRAM test on the printer, but no errors were detected.

Please advise.