The ports are not the issue otherwise you would not be able to connect at all. Chances are that the network connection is not good enough or that your host machne spins downs its hard disks or otherwise goes to sleep...
It must a be a network connection issue, because when I've tried to access the file from my office (WAN) I did not get the desconnection issue even if I was connected for hours. Since in my office the router has the 5003 port opened, this is why I thought that ports could matter.
I will try connection to server from another location and see the behaviour.
Everthing I've heard & read says that FM 13 is "very chatty" over the network with a shorter time-out for a non-response from FMS (or a shared FMP file) than we've probably experienced with previous versions.
Thus, any weaknes in the connection will most likely result in a disconnection.
We saw this last year when we tried to login to our served files in NZ from several North American campgrounds' WIFI services. The pipe was clearly not large enough to keep a connection up. Obviously, there was no way of diagnosing the cause of any disconnection -- it merely showed us how important is throughput through that pipe!
Thus every situation will be somewhat different. Good luck in finding, and then fixing, that weakest link!!
PS You should probaly also make sure that you uncheck (or is it check?) the No Sleep option for FMP (and or FMS) in the Info Window as has been discussed in recent TechNet posts.
It must be a mere connection issue not related w/FMP, since I do not experience the issue from other locations. We are working with our ISP to change router configuration.