Mail shows 100% usage for Gmail account, doesn't rebuild

For some reason Mail shows me 100% account usage on my Gmail account. I've deleted most mails with attachments, erased deleted items and did a rebuild. However, the account information still shows 100%, I don't get new mail and nothing seems to work. When looking at gmail.com, there's 1% used.

I've already removed the account and added it again, restarted the Mac and now I'm running out of ideas.

Anyone knows a solution?