Texteditor doesn't show content

I ask hear, because I don’t find anything on the net:

Actual behaviour

All over kanboard the text-editors doesn’t pick up the former content I saw. In the script-tag I find the content. But the system doesn’t paste it into the text-editor-textarea.
Expected behaviour

If I open an edit window, I expect, that the former content will appear in the text-editor
Steps to reproduce

Edit a descitption in a task.

The system:
Kanboard version: master.40304082bd34255bf244ea9cba78001dfdc5e20e || v1.2.18
Database type and version: mysql, 5.6.42-log
PHP version: 7.2.34
OS: Linux localhost 3.10.0-1127.10.1.el7.x86_64 #1 SMP Linux localhost 3.10.0-1127.10.1.el7.x86_64 #1 SMP
Browser: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:85.0) Gecko/20100101 Firefox/85.0

My Browser console show this:

Have anyone a hint or another idea? Anti-Virus an so on I stopped.

I’d start by disabling plugins

I disabling the plugins. Same behaviour.

This ist the aourcecode. You can see, that in the script-tag the content is. But not in the text-area.

Nobody a idea?

It’s very uncool nothing to edit.

If you use Apache as web server, make sure to disable mod_security as mentioned in the docs.

Looking at the screenshot above, there is some Javascript injected into the page by Kaspersky. This is most likely the problem. I don’t know if this is an antivirus or some corporate “web firewall”.