I am experiencing so problems with the Sixt website. I just wanted to book a car for the upcoming weekend. I check availability without logging in and they offered cars in all categories. When I logged in with my Sixt Card # again to check prices the website states that there are no cars available at this station. If I log out all cars are available again. Strange, especially given the fact that you have a reservation guarantee using your Plat card!

In the end I ended up renting with Europcar as their weekend rate is 30% lower than the Sixt one.