check the settings under 'security' (under file/manage): Is 'fmnet' for the specific account activated for those files?
In the sharing dialog, did you designate certain privilege sets as OK thus disallowing others rather than use the allow all checkbox?
There is also a question of sufficient connections. You may need to buy more.
For instance, if you have the minimal connections and open the two in question then you can't open any more.
If you have a lot of connections for the server, then the problem is probably in the sharing setup as I mentioned.
Or it could be something else...