I have same issue -
I have already two Web (as well as FTP,...) servers behind my router with port forwarding from Internet to HTTP/HTTPS
one is on 80/443 (back-to-back), another on 8080/8443 and I try to set the MBL on 8000/44300
(I have no difficulty in defining port forwarding on my router [Belgacom b-Box2 = Sagem F@st 3464])
I set 8000/44300 manually on the MBL and I also get the "Connected (Relay connection established)
Port forwarding failed to connect on port 8000 and port 44300 . (Error 31526)" error.
Ports are correctly forwarded as I can check using, for instance http://web-sniffer.net/, and connect to http://188.8.131.52:8000/ (184.108.40.206 is, at the time of writing, my external IP address) - the result of this connection is "HTTP/1.1 200 OK" - and I can see also the page returned) which prove it works.
So why do I (do we) get the Error 31526 message?