This appears to happen when a custom serializer is set on ClusterOptions; the problem appears to be that custom serializes should only effect user data (documents, query, etc) and not system data (cluster maps, error maps, etc):
Fixed
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As reported in forums: https://forums.couchbase.com/t/version-couchbase-net-client-3-0-0-can-not-bootstap-with-couchbase-server-6-5-1/25846