I don't think it is expected/intended to specify the protocol when adding the URL referrer restrictions, which should be as generic as possible, e.g. *.wuuzilla.at/*
If you find that still does not work then, currently, there is no alternative but to not use any referrer restrictions.
We are working on a solution to this.
Our experience with the API for server requests is that they appear to be handled inconsistently, or, at least, from our perspective they are, as we are unable to identify the reason it does not work in a few isolated cases but does in 99% of them.
So we are going to add a two-key solution for such cases, with a key that includes URL referrer restrictions for the client-based requests we make, and an unrestricted key for the server-based requests we make (where the key is not exposed).
I don't have an ETA for it but it is currently in development.
I am having this problem, also. It has worked fine in the past. Not sure when it stopped working. I tried making a new API key and also tried the suggestion above to not have https:// in the referrer. Still not working.
Any word on when an update will come out?