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