20 days ago - AndrewT - Direct link
No real idea, sorry, I'm not aware of this as a general bug.

Does it matter who is host?

Can the MP save be loaded in single player and proceed okay?

Any mods in use?  
5 days ago - AndrewT - Direct link
What happens if your friend hosts?

When you launch this game there are three MP options; have you two tried all three?