Yes, it's partly solved now: I tried the 'Find Your Ticket' option and then it showed a message that said it had to be linked, which could take up to 5 minutes. However, ever since I only get the message: 'We are sorry Something is not working in this area, bear with us while we try to fix this quickly. Please try again later.'
I tried logging out and in again, restarting browser, but I keep getting this message. So now I know the tickets are there, but still no option to change to a later bus (if that's an available option at all to begin with)...
The suggestion to just pick another bus later sounds a bit scary to me. I went to Glastonbury once before, in 2013, and I remember back then you really needed to go to the right bus at the right time. However, that was a bus ticket from a ticket+coach package. Don't know if that's different from the 'regular' National Express buses that go back and forth?