I remember running into this problem before, but I can't remember the solution.
AE CS5 is saying a file path is too long when I do a collect file. The image sequence in question is only 7 directories deep -- which doesn't seem too buried considering how AE wants to structure collected files:
/Volumes/HardDrive/ProjectName/CollectFileFolder/(Footage)/CompName/Pr ecompName/results/3Dimage_multipass0000.tif
Is it number of directories deep or overall number of characters in the entire path that's the issue?