as the subject says: there doesn't seem to be enough free disk space available to start windows. Elsewhere I was told to free up disk space on my windows disk. But how can I when I don't have access to the system disk?
Under great pain I managed to free up enough space on under macOS that is message went away and I was able to shrink the partition of the start drive from 197GB to 135GB. So far so good. This didn't have any influence on the free space in my macOS. Looks like Parallels still claims the space and doesn't release it. Parallels Desktop 18.3.3 that is.