I guess I found another way to use this extension with this bug. I saw in other threads, that the tabs are disappearing after making for example 6 tabs, saving every single tab in the form and then the module. As I reproduced this bug it happens after I saved 4 tabs and clicked the save button from the module at the end. But now I found out, that everything works fine if I use content -> module instead of content -> HTML.
Now I have to make every tab as a single empty module with the position "tab 1, 2, 3... etc." but I can live with that.
Maybe this works for other ppl too?