Cache Server error - "failed to move file in place"?

Does anyone know what could cause the following Cache Server error message?

PUT f63b58d16a6a5f49a86b98b2eeb89a02_20874242458b2082f6d2f4b04bd63cfe (size 2730455)
Failed to move file in place ./cache/Temp23861569-16bd-442c-a0bf-56128961686f to ./cache/20/f63b58d16a6a5f49a86b98b2eeb89a02-20874242458b2082f6d2f4b04bd63cfeError: EEXIST, file already exists ‘D:\CacheServer_Unity5\cache\Temp23861569-16bd-442c-a0bf-56128961686f’

I’ve checked and the Cache Server is well below its max size, there is plenty of space on the drive, and we have been getting some weird issues lately with people needing to re-import files that should already have been cached. It’s running on Windows using the latest version of the Cache Server (5.0.1) for Unity 5, and this issue occurs using the legacy cache (cache5.0 directory is empty).

Any ideas?

Has the same issue in Cache Server 5.1.1. I do a clean install then press reimport in my project and get this message! Why?!

Failed to move file in place ./cache5.0/Temp927f6aae-bf66-431d-903e-23fb256b00a4 to ./cache5.0/a9/ (Error: EXIST, file already exists ‘C:\Windows\system32\cache5.0\Temp927f6aae-bf66-431d-903e-23fb256b00a4’)

As I understand it, the cache server for Unity 4 is a mess and can get corrupted easily. We ended up nuking and rebuilding our legacy cache overnight whenever this happened.