Wha?? I was just publishing an app like always and received this error. At first, I thought it was because I had to reinstall VS2010, but it’s actually a simple certificate issue.
It turns out they expire. Go under your PROJECT PROPERTIES and look at the SIGNING tab.
In my case, the Expiration date had come and gone.
You have the option of turning off the signing, but instead I just clicked the “CREATE TEST CERTIFICATE” button and my build/publish went on without issue. I did get a warning about replacing my app with a different certificate version, but that was okay.