After installing the mixamo animation editor plugin, I was unable to click up a script and have VS2010 open up… it would always say it was trying for a moment, then drop back to the built in editor with a message about SyncVS not being available (VS2010 Ultimate)… Opening the solution from explorer also failed silently. Opening from a clean open of VS would work, but the moment that Unity updated the solution, the problems would start again.
I removed the Mixamo plugin and things immediately went back to normal.