On some HP printers, when I try to import a certificate to enable HTTPS for the printer's Embedded Web Server (EWS), I get the error message "The format of the file is invalid". This happens both (1) when I have created the CSR using openssl and (2) when I use the "Create a Certificate Request" option in EWS.
Updating to the latest firmware does not help.
Example model: HP Color LaserJet MFP M477fnw