"Max couldn't convert the max file to an FBX file!" error

i have a project with a single max file that holds all the geometry and the animations. everything was working wonderfully until yesterday when i did something in max. since then i get a "Max couldn't convert the max file to an FBX file!" error every time unity tries to import it. no matter what i try to do - this is what i get. the thing i changed in the max file was really small and nothing that should have made the file so unusable - i flipped some faces, moved a few vertexes around and created a box. oh yeah, i also animated the box. normally, with keyframes for position, rotation and scale. nothing too exotic.
exporting the file to FBX will be the last resort since i don't know of a way to replace the prefabed max file with the exported FBX and there is a lot of things happening that reference objects from the max file specifically and would be a pain to replace.

i have tried:

• deleting the object i last worked on

• breaking up the max file into separate file holding only a few objects each

• importing the file into a fresh max file

• doing the import into unity on a different machine

• spinning around seven times counterclockwise holding a chicken over my head (not really)

nothing seems to help. i am getting the same cryptic message every time, only sometimes it is accompanied with the helpful suggestion to export the MAX file manually to an FBX file. thanks, yeah, i got that.
any ideas as to what could be causing this? i'm on a tight deadline and any help would be most appreciated. also i would like to know what to be careful of when working on the MAX file so this thing would not happen again.

thanks,

amit