Thank you for your notice. I found something else which I do not understand. Checking the program files under "C/programms/Microsoft office/Office 14" I find the Exel programm with a last modification dated 26. June 2013. I believe since then the problem started.

My questions:

What happened in June? Was this an automatic update from Microsoft which created the problem?

Why do I find the folder /Office 14? What does the number 14 mean?

Thanks for your help.

Peter