Correct URL is here: https://forums.bitfire.at/post/9602
if I browse DAVdroid in the F-Droid App the latest version show here is 0.5.7-alpha while I could install 0.5.9-alpha from the F-Droid website. I expected to see the latest version in the F-Droid App.
You have to refresh the repository within F-Droid via the menu.
And maybe check it “Automatic update intervall”-setting