Hi @Naur, are you able to pin down which version of F-Droid was the last to work with it? You can download past versions from the browser at https://f-droid.org/packages/org.fdroid.fdroid/. Although the website is not up to date with 1.0-alpha0, you should also be able to install that version via the F-Droid client itself.
The big ones to test would be 0.102.3, 0.104, and 1.0-alpha0. If all of them fail to work (after a clean install) then it is likely a problem with fdroidserver and we can further diagnose the problem.