Okay 500 error.. could.. maybe.. possibly.. be due to a permissions error on your actual user account itself. Considering the issue with the quota you seem to have had just now it's probably the next best thing to check out. Having said that, you can't really check this out yourself.
You'll need to contact Support telling them that you are receiving a 500 error on that user account and could they double check the system & folder permissions on that particular user account for you.
It might be an idea to create a test.domain.com subdomain while you're waiting for a response, just to see what happens.
Maximum Cash Discount on any plan with MAXCASH