Partial success: By running the very old Maven 3.0.5, the PR builds actually work again: https://openhab.ci.cloudbees.com/job/PR-openHAB2-Addons/4369/
This does not help us for https://openhab.ci.cloudbees.com/job/openHAB2-Bundles/ though, since we require at least Maven 3.1 on that one…
And I still have no clue why this downgraded solves that issue…