I've been having this problem for the past 2 days. I've made two posts on the subreddit over those 2 days but no one has offered a solution or even commented. Restarting the client doesn't work, repairing doesn't work, and even reinstalling doesn't work.
I did see a post on these forums from September of this year when googling a solution, and the only response to that post was to update Adobe Flash and Air. My Flash is up to date and I don't even have Air on my computer so I don't think that was the solution.
This problem seems new. Never have I seen it before and the lack of Google results, the only relevant ones being posts asking for a solution in the past 2 months, also indicates that this is a new issue. I hope someone finds and posts a solution soon.