this has been a recurrent problem. For some reason Aiprint stops working and I have to restart the computer to restore it. Then, after a few weeks, it stops working again. The computer "can't find" the printer, and no printers show up in the preferences panel. It started on an iMac 2013, with 10.13, continued on 10.14 and now on an iMac 2019. So it is clearly a software issue. Something went wrong and now it just gets forwarded into every new upgrade, which obviously do not fix the problem. Printing works with USB by the way. Since nowadays there is no such a thing as "clean install", what can I do? Thanks.
Well, an update, if it helps in troubleshooting: I restarted the Mac and when the desktop loaded, the printer (that was "not found" before) came to life and printed the document.