Nextcloud 11 Update blocks login on Linux Mint 19

Help! I can’t access my calendar anymore… or do anything else…

Now when I go to login (just point my browser to localhost), I can’t login anymore, I just get a big button that says “Start Update”. Clicking that gives me an error that upgrading between major versions is unsupported. There’s a link to a help page there, but nothing on that page seems related to what I want to do. Which for now is just to log on.

I have a snap installation of Nextcloud on Linux Mint 19 xfce x64.

I think it’s nextcloud version 11 (there are folder names in the snap’s nextcloud directory that start with 11xxx, is that right?) and the ‘current’ directory is symlinked to 11891.

Is there a way to tell nextcloud that I don’t want to upgrade, I just want my old login screen back?

Many Thanks!