1) Open ReSharper | Extension Manager;
2) Find installed “ReSharper.StyleCop” plugin;
3) Remove it;
4) Install "StyleCop by JetBrains" plugin instead.
We unfortunately have two different versions of Stylecop plugins in the extension manager. The "ReSharper.StyleCop" version is causing issues.
The "StyleCop by JetBrains" version is maintained by us. We'd love to merge the two, but author of the original is currently not responding to any form of contact.