At runtime, Unity WebPlayer is unable to move cache files. Moving file failed - Access is denied.

At launch, my game downloads an AssetBundle. Once the download finishes, it unpacks it, caches it, and proceeds to the game. This has worked fine for approximately a month. I have made no changes to any relevant code recently - I have spent the past few days developing new content/meshes/textures, not coding.

Starting today, for seemingly no reason, I started receiving this error when the AssetBundle finishes downloading:

Because the AssetBundle is never unpacked or cached, the game breaks, and I can do nothing.

• I have not installed any new software.

• I am running Unity Editor as an administrator.

• My hard drive has several hundred free GB of space.

• I have not changed any permissions or added any new users to my machine.

• I am running a relatively new (<1 month) Win10 install.

• Everything worked fine for approximately a month before encountering this error. I have not made any significant code changes.

• OLD VERSIONS OF THE GAME STILL WORK. I can run my build from 3 days ago and everything runs fine. But when I roll back my code to 3 days ago and recompile, it fails.

• Upgrading the Unity Editor from 5.2.2 to 5.3.2 did not fix this problem.

• Updating to the latest version of the Unity Web Player did not fix this problem.

• Disabling Windows Defender (my only antivirus software) and Dropbox did not fix this problem.

• Manually setting the cache folder to no longer be read-only did not fix this problem. (It automatically reverts back to read-only.)

• The cache directories are not being indexed by Windows.

I am out of ideas and exasperated. It looks and feels like there is some other software interfering with Unity Web Player's ability to move things around, but I have no idea what else to try.