iCloud access

Hello, I have just received my new MacBook Air 2020 and I can't seem to access/setup my iCloud.

The computer gets stuck on the login forever.

Another interesting fact is: I've been having a hard time accessing old photos from my iCloud on my iPhone as well. The phone is connected to my iCloud account but the photos seem to load forever and I'm not able to download them (it seems to have stopped the back-ups as well). However, I am perfectly capable of accessing my iCloud from any navigator such as Safari and Chrome and I was able to sing out and in from iCloud on my iMac.

So it seems to be an iCloud problem. But why is it fully ok on my iMac, partially ok on my iOS (it connects but doesn't back up nor download from cloud) and impossible on the new MacBook??

I really don't understand.