Why are you not allowed to overtake here now?

Answer 1 is correct because a motorbike (or any vehicle) coming toward you makes overtaking unsafe at that moment.

In Germany you may only overtake if you can do the whole maneuver without endangering or obstructing oncoming traffic, and if you can return to your lane in time. On a two-way rural road that usually means you must have a clearly long, free stretch ahead. If an oncoming vehicle is already visible and would force you to “squeeze in” or brake hard during the pass, you must not start overtaking.

Rule of thumb for similar questions: even if the center line is broken and there is no overtaking-ban sign, oncoming traffic is a direct reason not to overtake because you cannot guarantee enough space and time to pass and merge back safely.