Its taken me all day to work this one out.

I setup my application for clickonce with all the key and security
sessions and suddenly studio could not open the solution...instant
crash.

After going down a few false leads I seem to have narrowed it down to
enabling clickonce security and then compiling (to generate manifest
file) while having the debug "enable visual sudio hosting process"
checked.

The next time I open the project it crashes. Switching off the hosting
process and re-compiling fixes it!

Infact, switching on hosting while the manifest file is there causes an
instant crash.

Anyone else come across this problem?