is Fortress Hohensalzburg fully booked for this month?

I'm supposed to leave tomorrow from Munich to Salzburg to visit the fortress, however, I just couldn't find any tickets online till the 31st of January (same thing when I searched 3 weeks ago), which is odd. People could still get in anytime and there's no cap on tickets, correct? I do know that the railway is being worked on till the 31st of January, is that the reason why all online tickets are unavailable until the 31st? I really don't want to drive almost 2 hours for nothing.