The website won't let us cancel any online for the past two days. We have to call and I don't have time to sit at work and call to cancel reservations or modify. Hopefully the website will be up soon. If I were you I would try to call so you can get reservations that you want. Then you can modify.
Mine is off and on both on my browser and MDE. I just keep trying and it eventually pops up right.I recently had the same problem. The MDE app never works for me for booking the +10, always have to go to the actual website. The website was also glitchy last week (well, let's be real here, it's always glitchy lol). I had to log out of MDE online, then log back in again several times (and cleared cookies once as suggested above) but was eventually able to book everything online. Good luck!