I´ve REX SkyForce3D, ActiveSky16 and PFPX.
I'm not sure why, but here's the answer from REX support over PFPX:
Developer: "We have temporarily disabled this function this mainly first to performance issues on the server as well as we are awaiting word from PFPX. We emailed them again on 5/29. No word yet."
Support/Developer "I'm sorry to hear you are having this issue. Unfortunately, we have been trying to contact PFPX, and they have ignored our mail. At this time, we will have to remove support PFPX in the next update. I wish I had better news for you. If you want to try to reach out to their support and see if they can provide better insight into the issue, that may be an option."
Another developer: "You're very welcome. If you reach out and they are responsive, please update us as we would love to support PFPX!"
December 14, 2018.
Believe me: I also expect a solution / fix for this.