Update: The problem can be present on just one of the printers and not the other(s), it can definitely appear without having had the system restart, and it has now also appeared on the third...

I have two printers, one of which is an Epson Artisan 1430 (very new, installed with current drivers and current firmware) and one of which is a Canon iP6000D (very old, with old, unsupported...