At least one file is missing the 'attribute' attribute
A file node that has been read from the .vbproj or .csproj file is missing certain attributes that the project system requires. Currently the only such attribute is the RelPath attribute, which specifies where under the project folder the file is located.
This error is most likely caused by editing the project file by hand.
To correct this error
Remove the affected file node from the project file. Also, if the file is still on disk, you could switch to show Show All Files mode (by clicking that button in the Solution Explorer toolbar), right-click the affected file, and select Include In Project.
Any file for which the RelPath attribute is missing will not be added to the project.