I'm having the same problem.
I deleted all sessions in Tools > Settings > Active Sessions in Plesk, and everything works fine. But a few minutes later, I had the same problem. When I reloaded the page, it was working fine, and then, a few minutes later, the error returned!
I have same problem:
ERROR: Plesk\\Exception\\Database: DB query failed: SQLSTATE[40001]: Serialization failure: 1213 Deadlock found when trying to get lock; try restarting transaction, query was: DELETE FROM `sessions` WHERE (`modified` + `lifetime` < 1746715193) (Mysql.php:79)...