On 3-10-2010 there was a lengthy discussion of this problem on this forum.
The name of the thread was "FileMaker 11 Runtime Error"
And the solution was cited as:
Since you have 2 users accessing it, I am assuming you are not using a run-time.
1. How are you installing it on the server? Are you using FileMaker Server, or just plain-vanilla FileMaker on a server box?
2. The file is open on the host machine (the server)? Correct?
3. How are the users opening the file? Sounds like they are double clicking a shortcut directly to the network volume and file. That is a big "no, no" with FileMaker. It will cause data corruption. With the file open on the host machine, use File > Open Remote to access the file safely.
Here are some other best practice items you need to consider: