I guess it's some kind of SQL Database error , After searching I found on reddit this answer from Theymos so I'am not sure if it's the same issue or it's just a default message for a lot of issues .
It was a technical issue while I was sleeping. It's fixed now.
How it happened was that Slickage was downloading a big chunk of the database for testing without turning off some SQL transaction stuff, so ~10 hours into their query, some configured limit was reached (probably the size of the InnoDB undo log), blocking all other queries. The disk wasn't actually full.
(You might be worried about the security of letting someone have access to the live database, but they are restricted via SQL permissions to only reading non-sensitive columns.)