The existing file system appears to have been damaged possibly because cfdisk has not adjusted (shrinked) the existing file system before changing the partition settings. In my case, this kind of thing I only dare to do with gparted if partitions contain file systems with data.
I would try the second option I mentioned above, as my last chance: to start a live-rescue and look that allows us to gparted, but I am not very optimistic about it
It's a marvellous feeling, right?
We thank Dave for his decisive contribution. For future occasions try to backup everything before doing operations of this type. This small script works very well for me:
https://github.com/cleverwise/cya
That allows you to backup even hot systems. Just mount an external disc in /home/cya and run the script with sudo...