Ok, weird problem this one. Never seen that before, but here we go.
During my build process I get the error from SA that it could not find my assembly (which is built using VS2010, SA is invoked as a build task on AfterCompile).
Looking at the obj/Release folder, there is really no assembly, yet it must have been there because of the copy my build step makes before passing it to SA.
"Something" must have deleted it. Using process monitor it shows clearly that its SA processes the file, does actually a lot with it, and then deletes it. Right after it tries to reopen the file and throws the error.
The SA project file settings are identical as all the other projects i my solution which build just fine.
I do have a procmon dump if wanted.