Since calendar is an app, but fundamental email service isn't, one thing that I found is that apps can interact in ways that are completely unintuitive.

For example, I activated the ncdownloader app, and it caused mail to stop showing emails, or I activated nextcloud music and it stopped nextcloud news from updating.

You should check your logs, because usually when there's a problem it will show up in there. The logs I'm referring to are in your administrator panel. It will be completely unintuitive as to what exactly is going on. The other thing that you can do is just pay attention to which apps you've installed, and if there are any that are a little bit unusual, just try to disabling them and seeing if calendar mail works after that.