Reply from support: "The issue is being investigated as we speak as it indeed appears to be affecting a number of our users.
Once this is resolved this should be automatically refreshed for your account and the functionality working."

Yes, I had similar thoughts, this seemed so silly that I was almost certain there must be a misunderstanding.

I wrote to support once more, and they agreed that rating a game without writing a review should indeed be possible. They couldn't give me an ETA for the fix, however, so let's wait and see.

Just writing to keep this thread marked as active. Although GOG support has acknowledged the problem, there's still no news about a fix or a timeline (or a statement about the number of users that are affected, that would be interesting as well).