Since a latter general system update instead of printing (even the cups testpage), the "hp" cups backend process and journalctl play "how many cores can we occupy" jamming the journal with
Jan 08 23:07:37 karnak hp: io/hpmud/musb.c 981: invalid bulk out endpoint
Unfortunately i don't print that often, but a simple downgrade of hplip to 3.12.10a-3 and libusbx to libusbx 1.0.12-2 (while i'm sure printing has worked after "2012-09-30 upgpkg: libusbx 1.0.14-1" and i'm actually quite sure it has worked with that hplip version) didn't help, so i wonder whether even the kernel may be involved.
(and yes, hplip-plugin 3.12.11-1 is installed)
There's also https://bugs.launchpad.net/hplip/+bug/1093312 which basically matches my observations (while this is an i686 installation here) though does not mention "does not print at all"
-> Can anyone here actually print on current hplip/usb?
Update: moving from hpiijs to hpcups solved the issue. Apparently hpiijs is no longer actively maintained / tested.