"Clone virtual machine
A general system error occurred: Failed to write to C:\ProgramData\VMware\VMware
VirtualCenter\journal\1398968714.70: Error writing file. There is not enough space on the disk. Creating snapshot of Virtual Machine"
I checked and there is plenty of GBytes of free space on that disk partition.
So that's yet one more bogus error message.
https://communities.vmware.com/message/1528814 says to reboot vCenter, which sorted the problem for me.