When I run the "Send Fax" wizard from HP Device Manager, and I try to add a file to the fax, I get:
Unable to create io-slave. Can not create socket for launching io-slave for protocol "file".

I get this error specifically on KDE desktop environment.
Kubuntu 20.04
HPLIP 3.20.6

Any insight on this?
I think this issue is also what is preventing me from adding USB printers through HPLIP. But network printers work.