We have been bound to make changes on http://www.plantuml.com/plantuml
because of performance issues.
Which plugin is your GitLab server using ? There is a slight change to be done in the URL generated by this plugin.
An additional string ~1 must be added in the URL generated. It's really a minor change.
But it is not clear to us which plugin is using your GitLab server.
Do you know which team is developping this plugin ? We will be glad to help them.