Thank you very much for the quick reply. The WordPress search in the navigation bar finds nearly everything, but does not find the names of the guest authors, neither in the author box nor their names mentioned in the text or in the author line. Now it becomes weird: If I deactivate the Molongui Authorship Pro-Plugin and just keep their basic plugin version activated everything is fine and search is listing the authors. It tried a different search plugin: same result. They told me it could be a plugin conflict and to test everything. That means deactivating GP Premium and all the plugins. I am not very fond of this idea.
GeneratePress does not offer a multiple authors solution, by chance?