When I applied the 0.42b update, I got the same error as OP, that I was trying to downgrade from 15.0.8.1 to 14.0.6.0. At the time, I just left it, hoping things would be okay.
After a few days I noticed some contacts weren’t syncing correctly across devices, dug around a little bit and saw this post.
Deleted the user-data for Own/Nextcloud, then re-ran setup and then re-synced devices back and everything was fine.