Thank you for your post.
FileMaker Pro will post that error message if the network connection has been lost with that client. With many applications, a file is opened over the network, and when changes are saved, the network is then accessed. With FileMaker Pro, it is constantly checking the network, because changes occur automatically and the data needs to write to the host often. If the network connection is lost at any time, FileMaker Pro cannot write the information to the host, so the error message is then displayed.
The airport utility in OSX displays connection information poorly.
There are third party utilities that do a better job.
Airport express units can be poorly positioned due to the fact they are most often plugged into a wall electrical recepticle. Better reception can sometimes be gained by using an extension cord. Using the utilities referenced above to find an optimal position.
This is expecially true if the link is from a basement to an upper floor. (wires are always better)