In the current version of Rocket.Chat the invite feature is impaired. Invite links can be generated as usual but a person following such a link without being logged into the service prior receives an error message instead of being prompted to log in. If the person is logged in prior to following the link, access to the channel is granted as expected.
Although the problem is known to the Rocket.Chat project since 8th Feb the issue has just been corrected in the newest major version (https://github.com/RocketChat/Rocket.Chat/issues/24444).
Since a new major version introduces changes and new features, GWDG Rocket.Chat remains on the maintained, previous major version for some time before conducting a version change.