403 Forbidden error on commit

I'm trying to commit but I'm getting the following error:

MKACTIVITY of '/proj/!svn/act/...': 403 Forbidden (https://svn.<myaccount>.beanstalkapp.com)

Googling gives me the two most common problems: not using https (I am), and case errors (neither my account, project or my username have uppercase in them, so I can't imagine that's the problem).

I'm using TortoiseSVN, if it matters.</myaccount>