My IO froze even without linking to another machine. Second player was controlled via maple adapter from the filter board. And it would only freeze between games. Never during. It’s a software issue.
It's random. It also freezes via GDRom. Keiichi331 tested it just to rule out all possibilities. It will freeze both for CF card reader and Netboot too. I even tried different roms via net boot. Lock, unlocked, PAL colour hacked.
It actually freezes for me without linking to another machine. I have 1 machine with 2nd player controlled via maple adapter on filter board. And it will only freeze at the end of a game, never during a match.