Is this a hack? WordPress Usernames of every website we have changed into one single name automatically?

Most definitely, all user names changed to admin I’m guessing ? I had this happen to an old website before. If available, I recommend recovering from a backup just in case they injected code in your posts / pages. I also recommend updating your plugins, templates and WordPress to the latest version and removing old unused plugins / templates.

Hata!: SQLSTATE[HY000] [1045] Access denied for user 'divattrend_liink'@'localhost' (using password: YES)