Perhaps the UPnP support for your router is not reliable. This is an issue with some models of routers. UPnP opens a NAT closed port temporarily when a program requests it.

Other things that might be affecting it. Many incomplete files & shares when LW is opening can affect the communication between LW & the router when using UPnP. And since LW 5 is slower opening than LW 4, LW 5 is more affected by shares & incomplete files than LW 4 was. Unfortunately LW 5 does not have the option to not connect automatically when opening the program. With LW 4 you had that option which would stop the flood of attempted connections with incomplete files at the same time LW was attempting to connect.

Birdy is more experienced dealing with LW 5 connection problems than I am.