In testing this with 3 different setups of 9.10, I’ve been able to assign F12 with impunity to other commands and have them work properly, as well as removing its assignment to any key. In the event of it not being mapped to anything, it doesn’t drop garbage text, it just doesn’t do anything at all.
You may try renaming and replacing your Meconfig.db with the default as well. Is this the same installation that was the upgrade from v8 as referenced in your previous posting? Unfortunately, for some people that upgrade has been hit and miss, working great for most but getting horribly corrupted for a few. I’m not sure but the level of customization and/or what specifically was customized and what add-ons were being utilized seem to be related but thankfully these have been relatively few. On the down side to that, we haven’t really had enough to see any kind of consistent pattern in the corruption.