I don't know the internals of that particular tranny, but there are some out there that use a safety to prevent too low of a gear from being selected.
In this case I'm guessing more toward a damaged or misaligned internal component in that first gear sync.
When you put it into gear at a standstill, is it really hard or does it just slip right in? Do you know how to "float" a gear in a downshift? If you do, be real gentle but try to float into first from second and see if it will go.