Issue with some notification mails

This email simply informs you that the nginx process does not exist anymore (killed, OOM, ...) and monit has restarted it.

Quote from gert445

Exactly, but this makes no sense if it is done after every start of the server. The server goes down at night either way. I didn't have this message before.

When this message is sent during the booting process, then starting the nginx service took longer than monit waits for the first check.

You can modify the delay with the OMV_MONIT_CHECK_START_DELAY environment variable. The value is in seconds and defaults to 30.
Finally, the message indicates that the system startup for some services has changed and now takes longer than before. You will have to find out for yourself why this is the case.

Quote from gert445

I have the feeling that this delays the overall start by the specified time, and not just the monitoring service. The server website is also very delayed.

Are you sure about this option?

Hmmm this sounds strange because all services are started via systemd and NOT monitd on system start. Monit is just monitoring them.

The phenomenon you are observing can only occur if none of the services monitored by monit are started by systemd and monit then starts them after the waiting time. Then the problem is that the services are started too slowly or not at all by systemd for some reason.