I installed the Fiddler version I mentioned in my first post and the Fiddler Script tab (the Script editor tab in Fiddler next to the Composer tab) was missing. I now deinstalled and re-installed it and now the tab is there.
As the tab was missing tried I thought I had to install the old FiddlerScript plugin. However I did not know that this does no longer.
Anyway now everything is fine, thank.