'Could not validate certificate signature' error (Antennapod, Gingerbread)

I am using Android 2.3.6 (Gingerbread) and AntennaPod Version 0.9.9.5.

When I am trying to access gpodder.net, either to login or search podcasts, I get the following error message:

An error occured:javax.net.ssl.SSLHandshakeException: org.bouncycastle.jce.exception.ExtCertPathValidatorException: Could not validate certificate signature.

I tried to clear the cache, the data, and ultimately uninstall and reinstall the app from the Play Store, but the problem persists.

Any hints about what I can do here?