I have the same problem on an almost identical system (21.5" 2011 iMac, 2.5GHz 20Gb RAM). I was snooping around Logic Pro 9.1.7 with a file saved from Logic 8 and encountered this same issue (which I don't recall ever seeing in Logic 8). I also tried saving the file from within 9 to see if there was some sort of version issue--no change,
I also do a lot of work directly in the piano roll and this is where it will suddenly turn light gray and become unresponsive, out of the blue. No clue as to why. When I call up the Force Quit dialog, Logic is indeed in red with "unresponsive" beside it.
riknvic I happen to have a few instances of the G-Player plugin (plays Gigastudio files), although I found that this problem happens even when there are no G-Player tracks. I also use an EMU MIDI 1x1 Tab (USB MIDI out to an external keyboard), but onot on the track I was working on at the time. Just wondering if you happen to have any of these, which might help to narrow down the issue (if it's not a Logic 9 software issue).
The only other possibility might be that I have unvalidated plugins (currently all disabled) from when I set up all my old plugins from Logic 8; don't know if it could have anything to do with it but I figure it doesn't hurt to mention this.
Very puzzling. I do save often, but even 5 minutes of work lost is a lot of work to redo.
Any ideas? Any other possible software conflicts that could cause this sudden crashing? I'd hate to have to revert to Logic 8...