Please stop re-publishing released versions
ELK 0.9.0 was published at least 3 times already (last time today, Januar 4th) to the eclipse.org project release update site. Please stop that.
It breaks mirrors of the repository, because the checksums of the newly published artifacts are not identical to the old ones. If someone with a mirror has a mix of old and new files (as it easily happens when the mirror is populated at access time only), then P2 complains about the mismatches and the build fails.
EDIT: After a colleague pointed me to the release plan, it looks like 0.9.0 is not even released yet. But https://download.eclipse.org/elk/updates/releases/0.9.0/ contains artifacts at least since December 7, because that's when I started mirroring that version. If that was never meant to be a release, then please change the build scripts to not publish to that location before the release.
ELK 0.9.0 is indeed not released yet, but will be once EMO approves. As you noticed, we usually supply a pre-release version of the update site to catch issues early on. This time this was quite good since we did indeed catch some issues.
For the next ELK version, we will try to do the pre-release at a separate location to prevent this issue.
Since we have now documented this in our release guide I think we can close this issue.