0 visitors on every 11th day of the month!

I have got the problem that the graphics show 0 visitors on the 11th of each month. Is there a way to fix this?

Don’t you have the CRON that fails on the 11tn of each month?
Check maybe on the log file…

I checked in the log file of my host… but there is nothing wrong. Maybe it is some setting in WP itself… I will keep on looking for the error.

In order to get the values back, you can invalidate the data for each missing date:
https://matomo.org/faq/how-to/faq_155/

How do you invalidate the data for a date?

Try something like: https://matomo-server/?module=API&method=CoreAdminHome.invalidateArchivedReports&idSites=1&dates=2021-10-11&period=day&token_auth=yourtoken