How to remove the message "Some extensions are missing from this product" in ReSharper Extension Manager

Problem:

When using ReSharper Extension Manager, there are some scenarios, when the following messages can appear and cannot be removed using the 'Restore' button:

• Some extensions are missing from this product. Click to restore.
• An error occurred while trying to restore extensions. Please try again.

Solution:

Go to %APPDATA%\JetBrains\ReSharper\v8.0\<VS_version> folder and remove all its content (usually, the 'packages.config' file).

Note:

As a side effect, all ReSharper extensions installed via Extensions Manager will be removed and should be reinstalled.