When this issue happens, what is logged in the CrushFTP log when you make an attempt? Anything?
If nothing is logged, then the connection didn't make it to CrushFTP because your IP forwarding is wrong, or a firewall blocked it... If the connection makes it to crush, crush logs a message about the incoming connection even before any login info is processed.
Then it has to be an IP changing I think...or a router issue. If CrushFTP is still working locally, something else stopped processing the connections to it.