I entered the diagram editor and it tells me that the version currently shown (version A) is not that which is displayed on the Confluence page (version B). What do I have to do so that the version shown in the editor (version A) is published on the Confluence page?
The message is most likely shown because there is an unpublished draft version of the Confluence page in which the diagram has been edited. If you publish this draft version, then the diagram changes, too, will be displayed on the Confluence page. In case you are shown this message having entered the diagram editor from the Confluence page view (i.e. not from the Confluence editor) you can publish this diagram version by clicking on Publish this version now. Alternatively, you can also make changes to the diagram and save them. It will then directly be published on the Confluence page.
In the diagram version dropdown no version is marked as "last published". Why?
There are two possible reasons. Either, the diagram is new and no version of the diagram has been published yet, or it has been created before your upgrade to plugin version 3.7.0-Enterprise (or higher) and not been edited since.