@windbags Have you tried peak4's "solution"?
When I had this problem, I just let Opera " wait for program to respond" rather than "end program"
Be patient, it can take several hours, or even overnight.
I had sync disabled since the update but I just tried signing in again and it worked for me, too! "Several hours" seems a bit extreme but I did have to wait 5 minutes on a sluggish laptop and 1–2 minutes on a less sluggish desktop PC for Opera to respond again. After that I was logged in and everything had been synced.
It may well depend on how many tabs you have open at the time it hung.
For various reasons, and it never normally causes issues, I was probably running with 100+ open at the same time,
I had a few tabs, as I always have several "tasks in progress" that need the info, but nothing like that many. I did try removing ones I thought I could easily retrieve, but after three goes I was getting fed up of it "not responding".
Waiting for minutes/hours/overnight on the "hope" of eventually getting a response is not the mark of a useful browser - especially when Windows 10 updates keeps forcing me to restart the system.