I need a workaround for Resharper when it says 'Failed to modify Documents'. Does anybody know why it does this and how to get around it?

Disable the Visual Studio source control plug-in.

1. Tools -> Options -> Source Control
2. Change "Current source control plug-in" from "Microsoft Git Provider" to "None"

I hade the same issue with Visual Studio 2013.
The solution was to add the project to version control (git in my case).

Now, you don't need make commits if you don't want to but simply add the solution to version control and the issue should go away.

What did the trick for me was unloading and reloading the project in which the file that fails to be edited resides under. (Just right click on the project, and select "Unload Project" from the context menu.