You mention they are at the end of the leg of the star: have you tried
moving one or both Plus systems to a different connector to see if that's
the problem? If the problem continues at another physical location and you
can log on to the server, we suggest that the problem lies with FileMaker
rather than the Macintosh or the network.
Because you can access the server; and print, copy files, and so on; we
assume that basic network integrity is good; and the problem lies with
FileMaker itself. It appears that you are exceeding the maximum number of
simultaneous users. We would suggest that you contact Claris for a
solution, if our suggestions don't alter the performance of these two
systems.