I have 3 sites all running Netware 6.5 SP5. Client is 4.20. It's
worked pretty flawlessly which is why I've avoided SP6.

Anyway 2 days ago I started to get lots of calls about prints not going
anywhere. If I looked at the NDPS manager, a lot of printers would say
they need attention some unable to ping, even though I could ping them.
Or they would say "printing" but they weren't. I've restarted that
server and that resolved it for a day. Unplugging the printer and
plugging it back in will often trigger stuck jobs to print.

This all happened shortly after I created an iprint client setting for
one location in iManager. The only thing i changed there was telling it
not to show the tray icon.

Sounds a bit like what people were seeing post SP6.