I had this recently after we moved a number of projects in the solution. I worked out eventually, that each project actual appears in solution file multiple times each with path information! So even though the path in the main reference of the project was correct it was wrong further down the file.
So go through the .sln file and make sure the paths in all the references of each project is correct.
For instance, the first reference for one of my projects is:
Project("{F184B08F-C81C-45F6-A57F-5ABD9991F28F}") = "ObexPushVB", "Samples\ObjectPush\ObexPushVB\ObexPushVB.vbproj", "{E3692A59-D636-48E8-9B57-7DA80A88E517}"
EndProject
In my case the path there was correctly updated. But then we have also for that project:
SccProjectUniqueName8 = Samples\\ObjectPush\\ObexPushVB\\ObexPushVB.vbproj
SccProjectTopLevelParentUniqueName8 = InTheHand.Net.Personal.sln
SccProjectName8 = Samples/ObjectPush/ObexPushVB
SccLocalPath8 = Samples\\ObjectPush\\ObexPushVB
So all of those paths needed to be updated too! After I fixed that manually all was well. (The sample there is after the fix BTW).