I assume your iMac was up to date when you created the backup? Then it will have been build 1182 or something like this, and the backup has that version.
The Roon Server version 2.0 (build 1143) production that you seem to have on the ROCK is not current. Probably the installer image is not getting updated all the time. Try to run an update of the ROCK first, before trying the restore from the backup. Through the Roon remote > Settings > About.
(With build 1148 they broke compatibility with older builds, here.)