Hi AoE team,
Currently Xbox live sign in is using internet explorer protocols which is not ideal in my opinion. Instead switching to something implemented in Sea of Thieves where it just uses the default browser. It should be easier to implement this as it has been implemented by other games.
→ I also test Games on proton (Arch Linux) and we are discussing this issue here Proton github. Gecko is the implementation for internet explorer calls and it is supported by proton but the game implementations seems to be using an old API.
AoE 2 DE runs great on proton and is stable tested with both UHD graphics and default ones 
→ Other users likely will also have less issues signing in as other major default browsers like Edge, Chrome, Firefox, Chromium handle it well. There are identical issues due to this faced by the players.