I followed the errata link and disabled my plugins (ServerDefaultPage). First I updated to 1.1.21 and after that I wanted to upgrade/update to 1.2.0, but during the installation I got this error:
After pressing "ok" the installation ends with the notice "FATAL" Exit Code: 1
I wanted to remove (backing it up of course) the content of the /etc/imscp/skel/skel but I wasn't sure about it. Better to ask for help instead of doing something wrong.