This is an added security feature in ownCloud 9 to sign all the changes to avoid malicious code or files in ownCloud releases.
However, due to a bug in ownCloud 9.0.0, this message appears by mistake when installing some official apps like the Antivirus app, this has been fixed in ownCloud 9.0.1, which hasn’t been released till the moment.
As a temporary workaround to stop seeing this message, just go to the version.php file, and change the release string from “stable” to anything else, e.g. “myownrelease”
Rescan, then the error message will be gone.