After doing a lot of research with your advice I think something really strange must have been happened (is this correct English?).
Yes, the program folder was and is Calibre2. The appdata folder is calibre.
You all have resured me that I can safely un-install and re-install calibre and so I did it. It worked out quite well. Everything seems to be in place
I'm happy about that. Thank you!
Windows Search was indeed not helpful. I found the folders on my own before, but I couldn't figure out a file that had caused the problems. Unfortunately I've made the update before I've checked the PATH. I'm sorry.
I still think this was really weird. I hope next week's update will work. I'll have a look on it. Now I've installed 1.31.
I'm delighted how much help I've got. Thank you to all of you.