Arena client won’t open - stuck at “Game is still in progress”

This first happened a few weeks ago when I first installed the Mojave OS on my Mac laptop: I’d log into the client, load into a game (any mode), and then be stuck on the “game is still in progress” screen because the arena client wouldn’t initialize - no old-school blue-and-gold bouncing dock icon, no full-screen game entry screen. When I quit the game client and restarted, I’d get the Reconnect button, but it did nothing.

About a week or so ago, I tried again. I got a bugsplat crash, but then the 2nd client popped open and everything was fine for a week...until yesterday. Now I’m back at the above situation, can’t do anything despite multiple fixes/reinstallations, and mad as hell because Riot blames Apple yet offered not even the possibility of a workaround. But come ON, there’s gotta be something that can be done?!