It’s kind of safe to ignore but it means that outgoing network connections are not working properly. Besides not being able to load system notices, this also means that e.g. refreshing people’s Gravatars won’t work.
Do you need to use a proxy for outgoing internet connections? If yes, make sure to set the necessary environment variables (such as HTTPS_PROXY) in the indico-celery.service systemd unit.
Also see this thread and the requests docs on the env vars you can use.