Summary
You configured the PolicyServer to automatically create an encrypted folder for all users (e.g. C:\Encrypt). However, the PolicyServer was configured again, recreating another encrypted folder with a different name (e.g. C:\Private). This results in having two (2) existing folders for the File Encryption.
Your tried to delete the encrypted folder that is no longer in used and get the following error message:
An unexpected error is keeping you from deleting the folder. If you continue to receive this error, you can use the error code to search for help with this problem.
Error 0x80070091: The directory is not empty.
Error 0x80070091: The directory is not empty.
Details
The only way to delete the unused File Encryption created folders is to remove or uninstall the File Encryption first from the machine. As a security feature, File Encryption will not allow you to delete an encrypted folder to avoid a possible issue with hidden encrypted files being deleted unknowingly.
As a best practice, avoid protecting a folder via policy, and then unprotecting it. This is the same with a production environment where you would have client machines that may or may not have content in the folder. Some files may be hidden to some users depending on the permissions and encryption key used.