Aha, I'd glossed over the bit in the instructions that said to put the IP in the address bar -- I'd punched in the actual domain name and gotten a 404; that combined with the fact that the installer process saying it was waiting for cloudron to start had me thinking something had gone wrong. It's working now. Derp. Thanks for your help.
I forgot to post a follow up on this thread. I'd like to thank @girish for solving my problem. He responded quickly and arranged to ssh into my server to diagnose. He's been very patient and professional and I really appreciate it.
Basically the problem is due to MySQL addons not properly starting/building, so all apps using MySQL error out and cannot start or install. Hope the problem has been resolved in the final 2.4 releases 🙂
After disabling apparmor, I saw that mysqld would just quit without showing any error message or log output 😞 As a final thing, I just ended up moving platformdata/mysql to mysql-old and recreated containers again. That made mysql come up and then we restored all the apps.
Still a bit crazy/worrying that we hit this bug atleast twice now and there is no clear resolution.