I had something similar happen a few months back on a PC Engine Duo (Black);
I was getting an alternating beige and magenta screen with no hucard, and a black screen with a hissing noise with the hucard in. One of the 90 degree angle pins that connects from the card port to the board had a rust deposit that ran down to the motherboard from something (condensation? errant electrolytic fluid? i have no idea). I carefully removed the deposit and cleaned the pin as thoroughly as possible, and re-flowed the solder to the pin, which ultimately fixed the problem.
Connection to the hu-cards resumed on boot, the CD-ROM screen is accessible but the drive/HOP-M3 has been and is still dead at the moment.