Answer
This level of persistent error typically needs the affected Search Domain dropped and recreated entirely, not just re-enabled or re-synchronized — recreating the Search Domain also re-grants any database privileges that may have been silently revoked along the way, which is often the deeper cause behind errors that survive a simple reprocess attempt. After recreating the domain and letting it synchronize, previously-errored/unsynced documents clear up as part of that resync, rather than needing to be resolved individually.