https://www.reddit.com/r/valheim/comments/xuonm4/steam_deck_connection_issues_to_dedicated_server/

Not sure if you checked around much, but I found two possible solutions from the reddit link.

Possible Solution 1 by Ilotan:
"Yes I have found two ways. First one is what I am doing.

I am on xbox gamepass and friend is on steamdeck. Launch the server from the xbox user. Then if the steamdeck user streams from their pc instead of running the game from the steam deck it works fine. Not perfect but if you have a good PC and internet it prevents the steamdeck from getting so hot and runs better.

Reinstall the game on the steam deck with the proton version (properties -> compatibility -> select a proton version.). Issue seems to be with linux version"

Possible Solution 2 by No-Weekend7331:
"Hey guys, I have found a workaround that works for my Steam Deck and I didn't have to disable crossplay:

Go to desktop mode.

Add the game binary (valheim.x86_64) as a non-steam game manually. The parameters for me ended up being:

Target: "/home/deck/.local/share/Steam/steamapps/common/Valheim/valheim.x86_64"

Start in: "/home/deck/.local/share/Steam/steamapps/common/Valheim/"

Go ahead and play!

This works in Gaming Mode too, if you use the non-steam entry you created for Valheim.

I know this sounds crazy, but I guess the default way Steam launches Valheim misconfigures it. Found it by accident when I was trying to see if there'd be any output in terminal when failing to connect.

Note: Had to lower the graphics as much as possible to avoid overheating and having my Steam Deck crash. It's insane how poorly optimised this game is."


Hope this helps