Error 21 Print Overrun
The print job is to complex. Simplify the job by removing font changes etc. On the III, IIID, IIIP, enable page protect if you have and additional 1 MB of memory installed.
On the 4/4M/4+ and 5: The print job is to complex, enable page protect from the control panel and from the software. You may need additional memory.
4L Same thing with to much data or the page is to complex. Simplify the print job. Turn the image adapt and page protect to ON or AUTO either from the control panel or from within HP Explorers Remote Control Panel.