Critical error when changing text

Hey!

My wordpress page gets critical error when I change text in settings. The whole page works normally, but all tabs in the plug-in settings to the right of the text tab are now empty.

Ett fel av typen E_ERROR uppstod p? rad 41 i f?ljande fil: /customers/f/c/1/xn--trundn-0xa.se/httpd.www/wp-content/plugins/my-calendar/my-calendar-settings.php. Felorsak: Uncaught ValueError: Unknown format specifier "," in /customers/f/c/1/xn--trundn-0xa.se/httpd.www/wp-content/plugins/my-calendar/my-calendar-settings.php:41
Stack trace:
#0 /customers/f/c/1/xn--trundn-0xa.se/httpd.www/wp-content/plugins/my-calendar/my-calendar-settings.php(41): sprintf('oktober 3-%03, ...', 'F j-%d, Y...')
#1 /customers/f/c/1/xn--trundn-0xa.se/httpd.www/wp-content/plugins/my-calendar/my-calendar-settings.php(725): mc_settings_field('mc_multidate_fo...', 'Datumformat f\xC3\xB6...', 'F j-%d, Y', 'oktober 3-%03, ...')
#2 /customers/f/c/1/xn--trundn-0xa.se/httpd.www/wp-includes/class-wp-hook.php(303): my_calendar_settings('')
#3 /customers/f/c/1/xn--trundn-0xa.se/httpd.www/wp-includes/class-wp-hook.php(327): WP_Hook->apply_filters('', Array)
#4 /customers/f/c/1/xn--trundn-0xa.se/httpd.www/wp-includes/plugin.php(470): WP_Hook->do_action(Array)
#5 /customers/f/c/1/xn--trundn-0xa.se/httpd.www/wp-admin/admin.php(259): do_action('my-calendar_pag...')
#6 {main}
  thrown