PHP Notice: Undefined index: site

Where are you seeing that error?

This looks like an issue when you save widget settings on a site that isn’t running multisite; I’m just uncertain why you’d be seeing it *a lot*; that seems strange.

The message I get from the hosting company is:
You have reached entry processes (number of simultaneously running php and cgi scripts, as well as cron jobs and shell sessions) limit 23 times

Going to the debug.log file in Plesk there are too many messages:
PHP Notice: Undefined index: site in /var/www/vhosts/hcds.gr/httpdocs/wp-content/plugins/my-calendar/includes/widgets/widgets/class-my-calendar-mini-widget.php on line 243

The PHP notice will be fixed in the next release, but it’s still strange that you would get that message a lot, unless you’re spending a lot of time updating Widget settings…

Fixing that issue probably won’t have any impact on the message from your hosting company; that sounds unrelated.

No, I do not change settings in Widget. I have it on the home page for quite some time.
The problem probably occurred after the WordPress upgrade.
Unfortunately, there is no option to send you the log-file. Within 24 hours, the error message appeared 187 times.
What can I say … I will probably try something else since there is no solution.
Thank you for your time

So, not an extreme number of occurrences, but unexpectedly high. Odd. Good luck.