Hi Caroline and welcome back–nice to see you again and sorry for the trouble!
A lot did change from 3.9 to 3.10 but much less between 3.10 and 3.11.
Here are the notes of what changed in 3.10. Definitely worth checking out. And here’s what changed in 3.11.
Do you have any custom templates or functions that might have broken in the update? That’s the first place I would check for sure and temporarily remove them to see if that make a difference.
The other thing I would suggest trying off the bat is that the template being used in Events > Settings > Display is set to the default events template — just in case there is a conflict with the theme there that can be resolved that way.
Let’s start there and see what we find. I’m hoping between the documentation of what’s changed and checking for custom changes we can avoid any potential theme conflicts and having to explore that route.
Thanks!
Geoff