Either that, or you can't login with the same user acount twice.
Try creating a new account with another name.
"File" - "Manage" - "Security"
Do be carefull before you go messing around there, and make sur to make a backup!
You can log in with the same account, even several times. The problem is that you need a separate copy of Filemaker Client for each user accessing the file.
That's what I was afraid of. Thanks Sorbuster.