Ok, so I imported the package with the .meta files and got the example scenes working.
What (again) gets me thinking about this: What if there is already another asset using the id of the .meta files provided in your packages? This also applies to other Unity packages of course. ¨
What I already experienced with this were broken texture links to GUI rendering etc. … broken serialization due to .meta file conflicts basically.
Re-importing everything may help here, but no way to say that now there’s no broken links anymore.