"I've heard that something Poccil did broke the Input Button Processing event command, but I don't know what or how to fix it. I can't help."
Upon further experiment, this seems to be correct, as it's present in every post and pre-Maruno changes, so if anyone knows what's going on, that would be good.
In trying to configure the PDA's scrolling options correctly, I ran into this error too, since pushing up seems to automatically decrease a value in whatever index you're using (like a list); the reverse applies to Down. Left and Right worked perfectly fine though. Dunno if it helps. We tried to look for where this was being configured, but didn't find much about it. Maybe the two are related?