I had very inconsistent results when testing this plugin as a publisher (not as a pipeline step).

Sporadically, the "... (Open)" link in the message was showing up as "(<nulljob/some-job/9/|Open>)".
The null part is the Jenkins root URL.

I don't really understand how this can happen because both the "Jenkins URL" under "Jenkins Location" and "Build Server URL" under "Global RocketChat Notifier Settings" are configured properly.

After some research I did find the following statement regarding the usage of new JenkinsLocationConfiguration() vs JenkinsLocationConfiguration.get():
https://issues.jenkins-ci.org/browse/JENKINS-52983?focusedCommentId=347707&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-347707
RC notifier is using the apparently "deprecated" new JenkinsLocationConfiguration() approach which should be switched to .get(), AFAICS.