Bug/Issue Multiplayer Connection-Problems [via Steam]

Hey pals,

first, sorry for my bad english ^^

So, the problem is:

Since 2 days or so, my GF and I can't play MP anymore..
(we're using the same WIFI and it works perfectly fine)
the thing is, everytime I join her via Steam I can't open chests, teleport or other things..
And the world doesn't fully load. At some time, after joining her via steam, I just "hang" in the air and nothing's loading.
She can't even send me an invite that we're on the same "team"..
and when I click on the "save and quit" button the game gives me "no response" and crashes.
Its the same when she tries to join my game etc.
My WiFi is good, because every other activtity like youtube, browsing or other MP Games work..
yes, we're both deinstalled the game and re-installed etc; but nothing works.
I dunno why this issue exists because it came out of the blue.. -.-
and it seems no one has the same problem.. because I didn't found anything on the internet so far..

thanks in advance.. o: