I have also this problem. You need install from this guide.
https://flatpak.org/setup/Ubuntu/
Proton on Steam don't install all package. I used experimental proton on Steam.
No he does not need this! 
Sorry that didn't work either. 
Do you put all commands ? Very fast.
Did you select the proton ge version in steam ?
Misschien maar even in het Nederlands. Wat heb je allemaal geprobeerd en gedaan ?
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Maybe we should tlak in dutch. What did you try and what did you do already ?
sudo apt install gnome-software-plugin-flatpak
Dat gaat vast makkelijker.
Ik heb Wine geïnstalleerd
Ik heb Steam geïnstalleerd
Ik heb geswitched tussen Proton versies
Ik schijn de flatpack van Bourne al te hebben zonder dat ik het wist.
Die van hem is niks, jij gebruikt de deb pakket. Flatpak heb je niet eens nodig.
Heb je met jou spel ook proton 5.x geprobeerd ? Op protondb spreken mensen erover dat het gewoon werkt
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You dont need the flatpak plugins because you are using the debian package
Did you try proton 5.x ? On the protondb people talked about the game that it just works
I think he want to help in some mysterious way,
hmmmm, strange
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Ja die heb ik allemaal geprobeerd maar helaas. Ik heb toch sterk het gevoel dat ik iets over het hoofd zie.
Zal mij er eens verder in verdiepen.
I will look into this.
Sorry don't understand. Cannot help
I know he is a dutch person like me and sometimes its easier to communicate then using english. My english is not 100% so thats why i proposed to swap language.
I translated my posts so people know what i talked about 
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Ok but if people will searching in future information about the same problem about this a game then a language english could be more enjoyable.
Tell me how a steam debian package needs flatpak plugins ? Because i don't understand this at all.
And that this the damn trick. 
It is working.
It was a registry configuration. Tricky job with Windows.
Thanks a lot for your help. I'm really happy now.
2 Likes
Good to hear this and i am glad it worked!