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What typically helps to fix it is deleting the Solution User Options aka "SUO".

VS up to 2013

In the older VS it is stored as a "hidden" SolutionName.suo in the same folder as the main .sln file.

VS2015 or later

In VS2015 the same data was moved to a "hidden" subfolder in the .vs folder under the same folder as the main .sln file.

What typically helps to fix it is deleting the Solution User Options aka "SUO".

VS up to 2013

In the older VS it is stored as a "hidden" SolutionName.suo in the same folder as the main .sln file.

VS2015 or later

In VS2015 the same data was moved to a "hidden" .vs folder under the same folder as the main .sln file.

What typically helps to fix it is deleting the Solution User Options aka "SUO".

VS up to 2013

In the older VS it is stored as a "hidden" SolutionName.suo in the same folder as the main .sln file.

VS2015 or later

In VS2015 the same date was moved to a "hidden" .vs folder under the same folder as the main .sln file.

What typically helps to fix it is deleting the Solution User Options aka "SUO".

VS up to 2013

In the older VS it is stored as a "hidden" SolutionName.suo in the same folder as the main .sln file.

VS2015 or later

In VS2015 the same data was moved to a "hidden" subfolder in the .vs folder under the same folder as the main .sln file.

Deleting the .suo file definitely does the trick. It is a hidden file in the same path as the .sln file

-- VS2015 (It has moved, see below)

I'm also using VS 2015. As Greg M. noted, the SUO file is now in a hidden folder called .vs within the solution folder. I was able to fix the error by closing VS, deleting the entire .vs folder and reopening VS

What typically helps to fix it is deleting the Solution User Options aka "SUO".

VS up to 2013

In the older VS it is stored as a "hidden" SolutionName.suo in the same folder as the main .sln file.

VS2015 or later

In VS2015 the same date was moved to a "hidden" .vs folder under the same folder as the main .sln file.