To clarify what my problem is I will describe the behaviour before and after the update.
When I pressed Edit button before the update the CK Editor starts and I was able to edit the page. Everything worked fine. This CK Editor runs not in page. It has a separate dialog where I can use the editor and switch to source code.
After the upgrade when I press the Edit button the CK editor runs as inpage editor. The editor toolbar looks like the CK Editor toolbar, but the source code button is missing and the encoding is corrupt.
Please see the screenshots attached - and of course thanks for your patience 
Before upgrade:
After upgrade:
Settings in the user profile:
