Error: white screen after pressing a month arrow (>)

This sounds like a possible jQuery conflict caused by your theme or another plugin. To check if it is, can you test with all other plugins deactivated and while running the default WordPress theme?

Nope, not working. I’m using an old twenty eleven theme and even without all other plugins activated it still crashes. First it shows a small kinda preview of the main page instead the calendar and then goes blank.

can you try to enable wp_debug or check your php logs to see if there’s any other error?

I’m getting no errors in logs. With wp_debug I’m only getting this:

Warning: mysql_connect(): Headers and client library minor version
mismatch. Headers:50544 Library:50621
in /var/www/new-hypnosis.ru/wp-includes/wp-db.php on line 1482

Notice: Вызванный метод конструктора класса WP_Widget считается
устаревшим с версии 4.3.0! Используйте

__construct()

. in /var/www/new-hypnosis.ru/wp-includes/functions.php on line 3457

(the latter is ‘wp_widget is considered obsolete, use __construct()
instead’)

that’s weird, another way is to use firebox-firebug or chrome console or network debugger to see if you are getting any javascript error or see the response since the calendar is using ajax.