Warning: Unresponsive script jquery.js

I’m using “All-In-One Event Calendar” 2.3.4 on a customer’s website: http://www.bertsweinexpress.de/?page_id=28.
The customer complains that he often gets the

Warning: Unresponsive script
A script on this page may be busy, or it may have stopped responding. You can stop the script now, or you can continue to see if the script will complete
http://www.bertsweinexpress.de…js/jquery/jquery.js?ver=1.11.3.2

in Firefox on different PCs. I never had this warning myself, but I have to handle that complaint.
Any idea what might cause this warning?

https://ww.wp.xz.cn/plugins/all-in-one-event-calendar/