Could not load type '' (blank) when ref .NET assembly

I've created an assembly that I want to be able to call from C/SIDE in the Role-Tailored Client in Dynamics NAV2013.

I've got this all working locally but when trying to deploy to the server I can't compile the codeunit that references the .NET assembly

I have a variable of type DotNet but when trying to choose a subtype I get this error:

Could not load type '' <-- nothing between the single quotes!

I've followed all steps in the MSDN guide (save exporting the types I want to use) I've got this working locally. I can actually get it running by installing the assembly into the GAC, but I don't want to have to do this when it should work otherwise

(the add in is also registered in the Client Add-in table and is strong-named with the correct public key token in the table)

These are the things I've tried:

* Building for .NET 4.5, 4 and 3.5 all of which worked locally but not on server
* Dropping the dll in the client and service add-ins folder
* Dropping the dll into windows/system32 folders
* Running everything as administrator (dev environment, service etc etc - I'm logged on as a domain administrator)
* Ripping clumps of my own hair out

All the assembly does is calls out to some webservices and a couple of other things, I just can't work out why I can't get it to work

I suspect it's permission related since installing to GAC seems to make it happy

Any ideas?

Thanks in advance