Hi Cyb,
This error is unexpected, although harmless. I’ve dug through the code and we shouldn’t ever be getting to this point for _user/ docs - The issue is described in https://github.com/couchbase/sync_gateway/issues/2151 but was fixed in Sync Gateway 1.5.
Are you able to reproduce this issue by issuing a _changes request through the REST API too?
Is it only seen for a newly created user, and only seen once per-user? Also how is the Sync Gateway user being created?