I was never able to dig down to the exact problem, but I found a solution:
The origin repository is on a separate machine, accessible through a mapped drive on the Windows network. When I did the clone by navigating through Windows explorer, the clone failed with the error above. But when I used Remote Desktop or logged into the machine directly with my network user and password, the clone worked fine.
The only thing I can conclude is that it's a Windows/network login permissions issue. And some of the digging down into the repo errors hinted at this as well.
Hope this helps someone in the future!
Update: someone else in the office was having the same issue cloning a repository from the network drive to their own workstation. Testing showed that:
- Everyone else was able to clone the repository to their own machines
- She was not able to clone with her Windows login on someone else's machine
- Others were able to clone onto her machine with their Windows logins
This seems to support the fact that there's some sort of Windows permissions issue, rather than a corrupt repository or other Git problem.