I recently had a problem with one of my projects in Visual Studio 2015. The symptoms seemed to indicate a corrupted .suo file, but I was unable to locate it anywhere I have normally found *.suo files previously.
One of the symptoms was this error message several times during the loading of my project:
I naturally went looking for the .suo file to clean it up, but it wasn’t anywhere I could find it. Thankfully, I used the Search feature in Windows Explorer to find it. The ultimate location?
[Project Folder]\.vs\[Solution Name]\v14\.suo
Note the .vs folder is hidden and the solution file is just named “.suo”.
Once I deleted this and all .user files in the solution directory structure, the errors went away and I was again able to save user-specific settings like startup project, break points, etc.