Starting Steam form command line, the trace is the following (probably the first few lines are relevant):

Could not connect to X session manager: None of the authentication protocols specified are supported
Could not connect to X session manager: None of the authentication protocols specified are supported
GameAction [AppID 1938090, ActionID 3] : LaunchApp changed task to ProcessingInstallScript with ""
esync: up and running.
wine: RLIMIT_NICE is <= 20, unable to use setpriority safely
GameAction [AppID 1938090, ActionID 3] : LaunchApp changed task to RunningInstallScript with ""
GameAction [AppID 1938090, ActionID 3] : LaunchApp changed task to SynchronizingControllerConfig with ""
GameAction [AppID 1938090, ActionID 3] : LaunchApp changed task to SiteLicenseSeatCheckout with ""
GameAction [AppID 1938090, ActionID 3] : LaunchApp changed task to CreatingProcess with ""
GameAction [AppID 1938090, ActionID 3] : LaunchApp waiting for user response to CreatingProcess ""
GameAction [AppID 1938090, ActionID 3] : LaunchApp continues with user response "CreatingProcess"
/bin/sh\0-c\0/home/gergely/.local/share/Steam/ubuntu12_32/reaper SteamLaunch AppId=1938090 -- /home/gergely/.local/share/Steam/ubuntu12_32/steam-launch-wrapper -- '/home/gergely/.local/share/Steam/steamapps/common/SteamLinuxRuntime_soldier'/_v2-entry-point --verb=waitforexitandrun -- '/home/gergely/.local/share/Steam/steamapps/common/Proton - Experimental'/proton waitforexitandrun  '/mnt/EXT4-SSD/Steam Library/steamapps/common/Call of Duty HQ/bootstrapper.exe' cod.exe\0
Game process added : AppID 1938090 "/home/gergely/.local/share/Steam/ubuntu12_32/reaper SteamLaunch AppId=1938090 -- /home/gergely/.local/share/Steam/ubuntu12_32/steam-launch-wrapper -- '/home/gergely/.local/share/Steam/steamapps/common/SteamLinuxRuntime_soldier'/_v2-entry-point --verb=waitforexitandrun -- '/home/gergely/.local/share/Steam/steamapps/common/Proton - Experimental'/proton waitforexitandrun  '/mnt/EXT4-SSD/Steam Library/steamapps/common/Call of Duty HQ/bootstrapper.exe' cod.exe", ProcID 62294, IP 0.0.0.0:0
chdir /mnt/EXT4-SSD/Steam Library/steamapps/common/Call of Duty HQ
ERROR: ld.so: object '/home/gergely/.local/share/Steam/ubuntu12_32/gameoverlayrenderer.so' from LD_PRELOAD cannot be preloaded (wrong ELF class: ELFCLASS32): ignored.
ERROR: ld.so: object '/home/gergely/.local/share/Steam/ubuntu12_64/gameoverlayrenderer.so' from LD_PRELOAD cannot be preloaded (wrong ELF class: ELFCLASS64): ignored.
ERROR: ld.so: object '/home/gergely/.local/share/Steam/ubuntu12_32/gameoverlayrenderer.so' from LD_PRELOAD cannot be preloaded (wrong ELF class: ELFCLASS32): ignored.
GameAction [AppID 1938090, ActionID 3] : LaunchApp changed task to WaitingGameWindow with ""
ERROR: ld.so: object '/home/gergely/.local/share/Steam/ubuntu12_32/gameoverlayrenderer.so' from LD_PRELOAD cannot be preloaded (wrong ELF class: ELFCLASS32): ignored.
ERROR: ld.so: object '/home/gergely/.local/share/Steam/ubuntu12_32/gameoverlayrenderer.so' from LD_PRELOAD cannot be preloaded (wrong ELF class: ELFCLASS32): ignored.
GameAction [AppID 1938090, ActionID 3] : LaunchApp changed task to Completed with ""
esync: up and running.
wine: RLIMIT_NICE is <= 20, unable to use setpriority safely
ThreadGetProcessExitCode: no such process 62425
ThreadGetProcessExitCode: no such process 62423
ThreadGetProcessExitCode: no such process 62422
ThreadGetProcessExitCode: no such process 62421
ThreadGetProcessExitCode: no such process 62300
ThreadGetProcessExitCode: no such process 62431
Game process updated : AppID 1938090 "/home/gergely/.local/share/Steam/ubuntu12_32/reaper SteamLaunch AppId=1938090 -- /home/gergely/.local/share/Steam/ubuntu12_32/steam-launch-wrapper -- '/home/gergely/.local/share/Steam/steamapps/common/SteamLinuxRuntime_soldier'/_v2-entry-point --verb=waitforexitandrun -- '/home/gergely/.local/share/Steam/steamapps/common/Proton - Experimental'/proton waitforexitandrun  '/mnt/EXT4-SSD/Steam Library/steamapps/common/Call of Duty HQ/bootstrapper.exe' cod.exe", ProcID 62424, IP 0.0.0.0:0
Setting breakpad minidump AppID = 1938090
Steam_SetMinidumpSteamID:  Caching Steam ID:  76561198078184350 [API loaded no]
ThreadGetProcessExitCode: no such process 62501
ThreadGetProcessExitCode: no such process 62503
ThreadGetProcessExitCode: no such process 62499
ThreadGetProcessExitCode: no such process 62494
ThreadGetProcessExitCode: no such process 62479
ThreadGetProcessExitCode: no such process 62470
ThreadGetProcessExitCode: no such process 62464
ThreadGetProcessExitCode: no such process 62458
ThreadGetProcessExitCode: no such process 62436
ThreadGetProcessExitCode: no such process 62433
ThreadGetProcessExitCode: no such process 62420
pid 62426 != 62425, skipping destruction (fork without exec?)
Game process removed: AppID 1938090 "/home/gergely/.local/share/Steam/ubuntu12_32/reaper SteamLaunch AppId=1938090 -- /home/gergely/.local/share/Steam/ubuntu12_32/steam-launch-wrapper -- '/home/gergely/.local/share/Steam/steamapps/common/SteamLinuxRuntime_soldier'/_v2-entry-point --verb=waitforexitandrun -- '/home/gergely/.local/share/Steam/steamapps/common/Proton - Experimental'/proton waitforexitandrun  '/mnt/EXT4-SSD/Steam Library/steamapps/common/Call of Duty HQ/bootstrapper.exe' cod.exe", ProcID 62424 
ThreadGetProcessExitCode: no such process 62445
ThreadGetProcessExitCode: no such process 62424
ThreadGetProcessExitCode: no such process 62299
ThreadGetProcessExitCode: no such process 62298
Game 1938090 created interface STEAMAPPLIST_INTERFACE_VERSION001 / AppList
Game 1938090 created interface STEAMAPPS_INTERFACE_VERSION008 / Apps
Game 1938090 created interface STEAMHTMLSURFACE_INTERFACE_VERSION_004 / HTMLSurface
Game 1938090 created interface STEAMHTTP_INTERFACE_VERSION002 / HTTP
Game 1938090 created interface STEAMINVENTORY_INTERFACE_V002 / Inventory
Game 1938090 created interface STEAMMUSICREMOTE_INTERFACE_VERSION001 / MusicRemote
Game 1938090 created interface STEAMMUSIC_INTERFACE_VERSION001 / Music
Game 1938090 created interface STEAMPARENTALSETTINGS_INTERFACE_VERSION001 / ParentalSettings
Game 1938090 created interface STEAMREMOTESTORAGE_INTERFACE_VERSION014 / RemoteStorage
Game 1938090 created interface STEAMSCREENSHOTS_INTERFACE_VERSION003 / Screenshots
Game 1938090 created interface STEAMUGC_INTERFACE_VERSION010 / UGC
Game 1938090 created interface STEAMUSERSTATS_INTERFACE_VERSION011 / UserStats
Game 1938090 created interface STEAMVIDEO_INTERFACE_V002 / Video
Game 1938090 created interface SteamController006 / Controller
Game 1938090 created interface SteamFriends015 / Friends
Game 1938090 created interface SteamMatchMaking009 / Matchmaking
Game 1938090 created interface SteamMatchMakingServers002 / MatchmakingServers
Game 1938090 created interface SteamNetworking005 / Networking
Game 1938090 created interface SteamUser019 / User
Game 1938090 created interface SteamUtils009 / Utils
Game 1938090 method call count for IClientAppManager::GetCurrentLanguage : 1
Game 1938090 method call count for IClientAppManager::GetAvailableLanguages : 1
Game 1938090 method call count for IClientAppManager::GetAppInstallState : 2
Game 1938090 method call count for IClientUtils::RecordSteamInterfaceCreation : 22
Game 1938090 method call count for IClientUtils::GetSteamUILanguage : 1
Game 1938090 method call count for IClientUtils::GetAppID : 24
Game 1938090 method call count for IClientUser::GetSteamID : 1
Uploaded AppInterfaceStats to Steam