@vojtech.myslivec was so kind to answer my earlier question via twitter. To make it easier for other people to find the solution via $searchengine I forward the answer to this thread:
http://api.turris.cz uses certificate from our self-signed authority (used only for Routers) and is obsolete now. You can browse packages on https://repo.turris.cz which uses trusted certificate from #letsencrypt. Turris OS uses this URL as well from version 3.9