After updating the Maridb from 10.1. to 10.5 that error appears.
What is to do?
My system: Nextcloud 21.0.7, ubuntu 18.04 and Plesk 18.0.40 - The nextcloud seems works.
i found the solution: Under plesk the cron job is execute every full hour. I executed it manally and now after every hour it automatically executes - so i think the problem is safed.