Beherit on "[Plugin: WP Fastest Cache] Cron job problem"

There is a problem with others plugin which use cron job - e.g. your plugin breaks cron in my plugin where I have this same schedule interval. You should change the function setRegularCron as it is in wp_schedule_event documentation:

To schedule an hourly event in a plugin, call wp_schedule_event on activation (otherwise you will end up with a lot of scheduled events!):

https://wordpress.org/plugins/wp-fastest-cache/

Another test - the problem is not only with function setRegularCron but with all wp_schedule_event in your plugin :(

I could not understand. Please contact to me via email: [email protected]

Which part You don't understand? You use wp_schedule_event not in accordance with documentation https://codex.wordpress.org/Function_Reference/wp_schedule_event

call wp_schedule_event on activation (otherwise you will end up with a lot of scheduled events!)

You need more details? What details? I think I wrote very simple what is wrong.

Please contact to me via email: [email protected]

For what? I give You all details. Tell mi what You need more and I will contact via email.

must I add register_activation_hook(__FILE__, 'my_activation'); ? are you talking about it?

You must change code where You using wp_schedule_event - wp_schedule_event should be used only on plugin activation as it is described in documentation.

now I got what activation means :) If we don't set the schedule on the activation, more than one cron jobs cannot work at the same minute?

If we don't set the schedule on the activation then a lot of scheduled events will just stop.

a lot of scheduled events won't work or they are deleted from cronjobs list?

They will not work ;)