Sorry for the delay in getting back with you. I actually missed this post.
Please answer one question for me as it might give us a clue. You have been a user for a while, so what changed on this development machine between when things worked and when it stopped?
This is one of the most frustrating errors that I have seen as I have figured out what piece is failing, but Microsoft could not give me any straight answers (or to this point ... ANY answers). I was excited the other day because I could debug it and I thought I would be able to isolate the issue....but it went away and started working again!
So it is definitely environmental. Because something within my environment changed that allowed be to get to the DTE.
I had the same issue with a Macro that we had created in house. When you create a macro, it hands you the DTE object...so...you would think that VS would give you a valid instance to work with...but in my case it failed. I moved it over to the Add-In and it worked (makes no sense). I also had the issue with the BO Mapper shortly afterwards (the machine I am working on is a new development machine...thus the newness of the issue). But after debugging and changing some things...it started to work. I didn't change any drastic, just rebuilt the extensibility solution several times (maybe like 20).
I will see if I can reproduce again. But this definitely environmental and we have not yet figured out the root of the cause...sorry
I will see if I can get it to happen again on my side.