cannot add public (unauthenticated) web ics calendar

I am trying to import a public agenda in Gnome Calendar, for instance :
  https://indico.math.cnrs.fr/export/categ/180.ics?from=-31d
When adding this URL in the Gnome Calendar settings, I am asked for credentials which should not be needed (wget or Firefox download the ics file just fine without them).

In fact, the same problem seems to have been already reported here :
https://askubuntu.com/questions/1100027/gnome-calendar-asks-credentials-for-public-ical
Someone suggests to use evolution as a workaround, but IMO this is still a bug of Gnome Calendar which is the default calendar for Ubuntu 18.04...

ProblemType: Bug
DistroRelease: Ubuntu 18.04
Package: gnome-calendar 3.28.2-0ubuntu0.18.04.1
ProcVersionSignature: Ubuntu 5.0.0-27.28~18.04.1-generic 5.0.21
Uname: Linux 5.0.0-27-generic x86_64
ApportVersion: 2.20.9-0ubuntu7.7
Architecture: amd64
CurrentDesktop: ubuntu:GNOME
Date: Fri Sep 13 16:12:25 2019
EcryptfsInUse: Yes
InstallationDate: Installed on 2019-09-04 (9 days ago)
InstallationMedia: Ubuntu 18.04.3 LTS "Bionic Beaver" - Release amd64 (20190805)
ProcEnviron:
 TERM=xterm-256color
 PATH=(custom, no user)
 XDG_RUNTIME_DIR=<set>
 LANG=fr_FR.UTF-8
 SHELL=/bin/zsh
SourcePackage: gnome-calendar
UpgradeStatus: No upgrade log present (probably fresh install)