New Battle.net update Version 14542 breaks Linux compatibility with Wine/Proton via Lutris or Steam and will only display a gray screen.
Workaround is to delete the update folder Battle.2.net.14542 under $wine_root$/drive_c/Program Files (x86)/Battle.net/ and then start the client again.
Update workaround: Set WINE_SIMULATE_WRITECOPY=1 environment variable in Lutris
Even with disabled autoupdates, the launcher itself still tries to update on my end. Fixed it the barbaric way, by clearing the contents of that pesky folder and doing
sudo chmod a-rwx Battle.net.14542