G'day GTê,
We see you're getting an alert about your subscribed calendars, and you've done some great troubleshooting already.
We noticed in your profile that you might be using a Mac mini. Is that right? If you happen to be using a portable Mac instead, we'd recommend trying to connect to a different Wi-Fi network and then test to see if you get the same result. Knowing this will help to rule out potential connection issues.
You can also boot your Mac into safe mode to test there. This runs a basic disk check and then prevents some items from loading when you start up, so it could help in narrowing things down further. Check out this link for the steps, which will vary depending on your Mac model and processor: Start up your Mac in safe mode - Apple Support
Once you start up in safe mode, test it out and then click Apple menu () > Restart to do a normal boot. Then let us know the results or check the link above for further steps you can take depending on what you find in safe mode.
Regards.