Filesystems, Disks and Memory

Hi
This is my first post hope someone can help
I develop and support a multi user application running on TRU64. All users were running the app sucessfully until one user tried loggin in again. This user, and all other users who subsequently log in, cannot find a particular file in a shared folder dispite the permissions not changing and other files in this folder being found no problem and any user who remained logged still being able to access the file. The file is opened and closed each time it is accessed.
The only thing I can think of doing at this stage is rebooting but am afraid no one will be able to access.
The really weired thing is I can still access the file no matter how many times I log in.
I and the users are in the same group although I do own the file. The permissions are -rw-rw---
any thoughts greatly appreciated

What is the group on the file? Maybe the other users are no longer members of that group.

The groups haven't changed, that was about the second thing I checked
The things I have ruled out are permissions/ownership and paths. Also the app hasn't changed, no new version implemented for weeks and the physical file structure has not been altered

Simply saying you ruled something out is not very informative. If I had a penny for every time a poster incorrectly ruled something out I would be a rich man (well, I could afford to go to a movie and buy some popcorn anyway). What technique did you use to verify that group did not change? Try changing the file to 666, can they access it then? Are the other users prevented from accessing other files 660 with that particular group? Do you have the "groups" command? If so, run it as those other users. What is the timestamp on /etc/group? Were these other users recently added to any other groups? How many groups are you trying to add them to? Is that greater than the maximum number of groups per user for your OS?

Problem solved
It was down to paths, which I know I said I had ruled out, but it was triggered by a ISAM index becoming corrupted.
Some of the users had a complete path set to a working directory in their profile and some did not. The reason for this is due to the development of the app which now sets all necessary paths for its operation, so older users have the complete path newer ones do not. One of the older users was not effected by the index corruption, for some reason, all other users were. This led me to believe that the app had set the paths, but this turns out to not be the case. I don't know why this one user was not effected. I just needed to swap 2 lines of code to ensure the correct error is returned every time. Thanks for the replies anyway