All I get is a 404 error.

Either the key should be made available under that URL or the installation instructions should be adapted to point to the correct location.

Would-be users affected by this issue can alternatively obtain the key from either of the following URLs:

• https://obs.kolabsys.com/repositories/Kolab:/16/CentOS_7/repodata/repomd.xml.key
• https://obs.kolabsys.com/repositories/Kolab:/16/Debian_9.0/Release.key