Bastogne was quite the uphill battle. I spent weeks wrestling with the editor having it constantly crash on me. You could try a 'mass weld' of all vertices in 3Dmax (if you use it). That seemed to do the trick for me. But I also spent weeks cleaning up the navmesh and deleting areas I didn't think the bots would ever go, or weren't important to the gameplay of the map. So, I don't know what I did exactly but once I had a bug free version of the mesh working ingame I kindda decided not to touch it anymore.
Vossenack is deceivingly not ordinary. It looked like it would be easier and less trouble than Bastogne, but it isn't. It took longer to generate than Bastogne (Bastogne = 74 hours 56 minutes and 32 seconds, Vossenack = 135 hours 16 minutes and 14 seconds) and seems to have just as many issues. And unfortunately none of the lessons I learned from Bastogne seem to be helping me with Vossenack. So I temporarily moved on to Hurtgen Forest, because I don't have the energy for another uphill battle right now.
Vossenack will crash when it reaches the generate GTS Data portion of the create navmesh process because there is an object on the map with collision mesh issues. I believe on Vossenack it is the vossenack church windows that cause this problem. Just delete them and it should generate fine. Be careful I believe there are a few static objects that somehow ended up on the gameplay layer (layer 2), like half the trenches in the little trench system on the map. I'm not sure if the generator takes these into account but if you haven’t caught this already I suggest making sure all statics on the map are on layer 1.