So I have never seen this issue before and don't know where to begin troubleshooting the issue. 

```
Cannot register assembly "obj\Debug\MyProject.Vsto.dll".
Loading this assembly would produce a different grant set from other instances. 
(Exception from HRESULT: 0x80131401)  

```

Where do I start to troubleshoot this? 

I should mention this is a VS2008 environment with VS2008 SP1 and this solution is:  

1. A VSTO Excel 2007 Project (C#)  

2. A Data Access / Service Layer DLL (C#)  

3. An MbUnit test project for #2 (C#) 

**UPDATE:**
I should add this did work fine for several months. The only thing that I have changed in the last week or so is I have started working on the code via Team Foundation Server (TFS). 

**UPDATE 2:**
Deleting the `.suo` file worked for a little while. Now I'm getting same error again.... hmmm. Guess I'll close the project delete the `.suo` again.

**UPDATE 3:**
VS2008 will allow me to compile the solution once. The second time I try I get the error.
If I exit, delete the `.suo` file, and reopen I can compile one time again. Any thoughts to the cause? Is this an VS2008 SP1 thing?

For the bounty I'm looking for the permanent solution.