Somewhere at the end of 2018, the Baïkal calendar synchronization on my iPhone started to show some problems. Continuous synchronization in the background did not seem to work.

By changing some of the Baïkal account settings on the iPhone, the synchronization could be made to work again. Intentionally, I create invalid settings, let the validation pass and save the settings. Then, I restore the original, correct settings again. As a result, the iPhone starts a full synchronization, which takes a few minutes in my case.

Further analysis showed that probably iOS does not like to synchronize using a self-signed certificate on the Baïkal-server. There is no error reported. Other users with comparable issues have reported this in discussion groups.

Some research led me to trying LetsEncrypt. For my server setup, there is a script available for using Certbot, a tool that automates issuing and installation of LetsEncrypt-certificates. Worked like a charm. The Baïkal-server now uses a real certificate and iOS synchronization works again.