Multiplayer connection problem

[Xeanoa]

Over the past half year I've been playing Factorio regularly with my partner. Over LAN when she's home, and over Internet when she's at the university.
That's always been working fine until Monday, when she tried to connect to the game. After a couple seconds, she gets 'server not responding' messages, and my client shows her being dropped from the game. That happened as soon as during the map download phase, but never later than after a minute.

On Tuesday, she was home, and the game worked just fine over LAN, no issues whatoever.

She's back at university now, and again we're having the same problem. She connected to my game, and after a few seconds or a minute she's being dropped from the game.

Everything else, including pinging each other's IPs, speedtest, VoIP, it all works fine. [Except Skype, so we're using Discord. But Skype problems are nothing new to me, everyone I know who uses it has problems with it...]

I'm totally at a loss what the problem could be. Does anyone have a clue/idea/suggestion what to try?

[Loewchen]

Post the corresponding logs from both parties, see my signature on where to find them.

[Xeanoa]

Ok, so this time (second attempt) the game behaved somewhat oddly, for the first time giving an error message (client side) on one of the connection attempts.

[Loewchen]

Something is changing the port of your friends address every few seconds so the packets are not recognised as coming from her.

[Xeanoa]

Something is changing the port of your friends address every few seconds so the packets are not recognised as coming from her.

How is that possible, and why doesn't it affect other services? She's connected over the university/dorm network, and we never had problems before. Is this something we cause/can fix ourselves, or do we need to contact the University's IT-service?

Result for reference
Speedtest