I keep getting emails that my Dropbox is full even though it's empty

I keep getting these emails that say "you’ve reached your storage limit" and "Your files have stopped syncing across your devices because your Dropbox is full".

In reality, my Dropbox is empty. When I log in and check my Subscriptions page, that also tells me that I am using 0 bytes.

There is something confusing though: I have Dropbox Basic which gives me 2 GB space. Still, the Subscriptions page tells me that I'm "using 0 bytes of 5.25 GB". What is that 5.25 GB?