BTW, the error I get before reconnecting is 'RPC Failure'
You are possibly referencing the wrong Interop files.
If you are compiling against .Net 4.0 you get these from C:\Program Files\SolidWorks Corp\SolidWorks\api\redist
If you are compiling against .Net 3.5 you get them from C:\Program Files\SolidWorks Corp\SolidWorks\api\redist\CLR2
They should work for 2013 and 2014.
By the way, what do you mean by "API crashes"? Do you mean your application, or Dev Studio?
I'm not using .Net, I'm using OLE/Active
I create an instance of the interface to the OLE object and after trying to open the 2014 document on 2013 (SolidWorks->OpenDoc6(...)) I get the appropriate error but after that, any method call to the instance throws an RPC Failure error.
My app and the dev environment are fine. I have to destroy the API interface, recreate and reconnect to SolidWorks.