Modify second time nothing works.

Good afternoon,
It looks like a nice frontend-editor but I have a weird problem.
When I start editing a page and change for example a word to bold it works fine. When I then go to another line and type text that also works. When I then want to change a word to bold, nothing happens. All the functions on the editor don’t work anymore. I can click on them but nothing happens.

When I navigate to another page and come back I can change something and the whole problem starts again.
Hope this is clear.

Site is up to date. I disabled all plug-ins, just this plug-in enabled.
Also I changed to the TwentyFifteen default theme.

What could be the problem?

https://www.ads-software.com/plugins/wp-quick-front-end-editor/