Maybe roads created with some mods will cause issues even with the mods disabled, because of mod data embedded in the map?
The first error seems tied to 'Traffic Manager + Improved AI' etc. which builds on Traffic++, and the second appears with 'Fine Road Heights', and both even in new maps.
Actually I'm quite surprised by the amount of mods that DO work with Snowfall, those two are the only two that I've had to disable.