My problem is now solved thanks to SuSE support. I send them the Siga output and I suppose there they found out what was wrong.
I didn't have time to post it so far but here it is :
Most likely, it's a coincidence that the problem occurred after first using the memory stick. Most likely, the online update did not update all packages, only some of them.
Please ensure that all the updates for YaST2 are installed. The update links are below. Download and save these files to disk, for example the /tmp/ directory, and then open up a console/shell. Change the user to root using:
rpm -Fvh *.rpm
And this should install the updates for YaST2. Now try to open YaST2. Do you still have problems?
After I followed the instructions, I got a "dependency" error about HWINFO* packages missing. I downloaded and installed them as well. Now all is working again and I could do the update.
Maybe this help to you as well.
For your info, This all started after I attached an USB hub & memory stick.