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 ā€œStart Updateā€. 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 ā€˜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!
Brian