Gnome Calendar and Contacts do not syncronise with Nextcloud

I am running F37 on a laptop and on a desktop computer. The laptop’s version was updated from F36 whereas the desktop has a fresh install of F37. On my laptop Gnome Calendar and Contacts work perfectly. On the desktop instead, Calendar and Contacts do not syncronise with Nextcloud. I correctly set up my accound in Settings > Online Accounts but they do not show up in either app. I get however this tray icon, which I have no idea what it means, when I run Calendar. Any clue?

I have some updates to this issue, which progresses weirdly.
I had to re-set the nextcloud account also on my laptop (for unrelated reasons). Now it does not work there either.
First of all: it does not make a difference if I write the server address with http, https or neither of the two. Only difference is with https that gives me an option to accept an untrusted certificate. The result however is always the same. No calendar, no address book. Neverthesess, the folder appears mounted and accessible in the file manager.

I also installed Planner from flathub, which appears to recognise my nextcloud account but it is unable to retrieve information from it. The very weird thing is the following. If I add a nextcloud account manually in Planner, I get a prompt to save the credential in gnome-calendar keyring. After filling in the details, the task lists appear in Planner and the calendars in Gnome Calendar… (no luck for the address book though). The very weird thing is this. If I do not set any account in Gnome Settings, I have no reading in Planner (which is expected) but… if I add an account in Planner, I get the calendar keyring prompt and then my calendars appear also in Gnome Calendar (even if there is no account known to the system)

This is not an issue with my account, since I have two nextcloud instances on two different providers and neither appear in contacts and calendar after adding them to Account Settings

Any help on this issue would be greatly appreciated