I know it's been 1 month+, but I'm a little triggered by that issue being still present right now.
(You should try Naaackers' idea, verify game files in your Uplay.)
Did you get stuck backing OUT of the game(Laus' situation) or did it happen in-between rounds, after you chose your operators and proceed to loading forever? If it's in a loading-in screen, can you hear the in-game audio?
- If not, wait or ALT+F4.
- If you hear the audio and you REALLY don't wanna lose, the only way is to wait - you should hear the audio bark whenever someone leaves the game, when the other side is empty you'll win the match automatically.
It's a pain-in-the-butt issue. Very hard to repro, happens in various environments and situations -- and when we have it we're on a build without logs. I'll try to track it back at work, and since it happened to you on Season 3, I'll go try to trigger it on the previous Master Candidate.
How you could help:
In the meantime can you boot your game and once in the main menu, press F10(Settings), and turn the 'Display Game Info' ON.
Can you confirm that you see the version you are on (bottom left), right now for S4 it's v:10211195
If it happens again, let me know, I've been wanting to find the repro steps for months.
You can reach me at work by e-mail if you get any info for me: firstname.lastname@example.org