Musicbee 2.5.524
If you have a period in the auto playlist name, it will not sync to ipod classic. (Example playlist name: Rating 4.5)
The sync screen says "Object Reference not set to an instance of an object."
Renaming it without the period worked. To confirm this was the issue I added a period to another playlist that was working, synced that, and it too failed. Then renamed it back without the period and it worked again.
This may be due to the period being a property delimiter in C#