Can no longer create VMs: This folder already exists and possibly belongs to another machine

[AnotherHowie]

"Cannot create the machine folder _ in the parent folder_.

This folder already exists and possibly belongs to another machine."

The underscores aren't really there, but there is a space in those two places. I think there are supposed to be pathnames there. The default path for VMs is already set, and I have created dozens before now.

The VM names I'm trying to use are not already existing folders. I just upgraded from a recent 4.2 to 4.3.4 with no change in this behaviour. Existing VMs run fine though. I've tried to create a windows and a freebsd vm, just in case it made a difference (it didn't). Doesn't matter if I choose to create a disk or not, either.