Hi all,
With Sonarr installed as a container (from linuxserver.io), choosing host enables you to share the host’s IP address, but due to some bugs (since the Synology docker version is really outdated) some push/pull network requests get timed out.
this caused series lookup to work fine, but trying to setup notifications with telegram/pushover failed.
the easy solution was to re-deploy the container in bridge mode and voila, everything works again