I got the same error.
There have been reports for many months of similar issues:
When reaching out to Support via ticket, I was immediately told it’s a known issue and there’s no workaround. I’m trying to keep the ticket open to see if a manual reset is possible.
I have quests for rewards that require Duels and this is a perplexing situation that I don’t think I ever came across, an actually unplayable mode. I know it says “Beta” for a reason but it seems abandoned.
I suppose the major thing to fix is to remove Duels as requirement for quest rewards, at the least.