[Mashiro]

Have you checked the logs on your server? Specifically all web server logs, possibly also PHP and most important the MySQL server logs (whether it is MySQL or MariaDB or another solution doesn't matter).

Have you tried restarting the MySQL server and the other web server components? Maybe this will help? Have you checked the overall state of your server? Enough RAM? Is disk space in good state?

Opening such a thread with just this screenshot... it could be anything. The error is absolutely useless without any detail. It used to be that MyBB would show the exact SQL error message and the SQL statement. Nothing here. So maybe something is seriously wrong with your MySQL server.