Unfortunately it didn't solve the problem, but I have replaced Firefox with Vivaldi and that has solved all my problems
So I consider the problem as solved!
Update to my previous message: It seems to have an impact which browser I use. I normally use Mozilla Firefox, running on Linux Mint, and with it I have the effect described in my first message. When using Google Chrome instead, the deletion of the event images seems to be working, exactly as it did ...
I have been using Zoneminder for years now. A few days ago I decided to upgrade from the previous version (1.29.x) to version 1.32.3, using this procedure: I reinstalled CentOS7 from scratch. Updated CentOS7 with LAMP-Server, and finally installed Zoneminder 1.32.3 from RPM-Fusion.org I configured 3 ...