Since 2.341, the sidebar icons of the gitlab branch source plugin are broken:
Restoring to 2.340 and below resolves the problem.
- Create a new multibranch pipeline (workflow-multibranch-plugin)
- Branch Sources -> Add Source -> GitLab Project (make sure you have the gitlab branch source plugin installed)
- In the "Owner" field add your GitLab username and the project you want to add in the "Projects" dropdown. Preferably you have a repository with a dummy Jenkinsfile, e.g. https://gitlab.com/NotMyFault/jenkinsfile to utilize the option "Jenkinsfile" on Build Configuration -> Script Path
- Save the configuration, wait until the scanning is done and you can see the broken image on the sidebar of the job overview.
This issue is a copy of https://github.com/jenkinsci/gitlab-branch-source-plugin/issues/185 to track it on the ux regression dashboard on Jira.