[CarlitoBrigante]

Not pointing fingers, but talking out of personal experience. You should also check the background of all the techs that you allowed to work on your server. In rare cases, some people will grab a copy of the databases and resell them or reuse them themselves - a database with a good amount of users and threads can be resold on websites like Flippa for thousands of dollars. I would start looking from the cheapest techs you ever hired, and move up from there.

Or, if your server is not secure enough, your machine might have been compromised without you ever noticing it.

In any case, whoever did it committed a serious crime, and you should contact authorities working on these matters in your country.