ok, so I tried a few things: Safe mode tip: didn't help installed latest Catalina updates: didn't help finally I tried the "create new >10Gb disk partition" tip from Nathaniel Gibson above and also installed the latest security updates and this seemed to do the trick and then it worked. I suspect that the creating new partition somehow forces the OS to clean up disk storage issues while it is resizing the partitions and this fixes whatever the problem was.