Here is a workaround that helped me:
Go to Debug-> Attach to Process. If the "Attach to" field says "Automatically: source code", click the "Select ..." button. In the Select Code Type dialog box, change the Automatically Detect Code Type for Debugging option to Debug These Code Types and select ONLY options for the project. For me it was "Managed (v2.0, v1.1, v1.0)." Click OK. Then click Cancel in the Attach to Process dialog box.
At this point, I can now do F5 Debugging.
Hope this helps!
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