Can't log on to client's website - Third party cookie error

I see this is a common problem with Webflow, but can’t seem to find a definitive answer.
When I try log on to my client’s site I get this message:

We’re having trouble with your browser’s cookies. Make sure your browser accepts third-party cookies.

One person from our team has managed to log on, but I can’t.
I have cleared the cache, allowed 3rd party cookies and added them to the list of safe sites, but none of it has helped.

Any advice?