https://bugs.winehq.org/show_bug.cgi?id=49446
--- Comment #3 from simms1@web.de --- I was in contact with Riot Support about this and (no surprise) they just forwarded me to WineHQ.
From the discussion with them I didn't gain any additional insights on this
error (just doing reinstalls or Hextech Repair tool didn't fix anything).
The workaround juliansperling@gmail.com described, did work for me (wine 5.6 staging).