"Connection to the host timed out"

[Karl Toffel]

I played Dungeon Defenders long ago and used to play it again last week with some friends who bought in on sale
Onfortunately, we can not play online - no matter who hosts the game (Ranked and Open) the "Connection to the host timed out" and we can not play together.
Sometimes we can connect, but then everyone - except the host - has massive lags and can not play at all untill they disconnect due to a timeout.
What happend to the game in the last year?