the Simlogical door controller -- the one that let's you put in colored fire alarms and control access. This is my chief suspect, since I remember installing some sort of global doo-dad, but I don't know where that doo-dad is to make it go away.
This would be my chief suspect -- anything that globally affects doors would cause a problem. I think all of those simlogical hacks started with the letters 'ij', so look for anything with that name in you downloads folder. If that doesn't help, you can here:
http://www.nectere.net/smf/index.php?PHPSESSID=d141881b342e8ac7f0fb2b7189e8bec3&topic=86.0 and download and check the original zips to get the exact file names.