NotMyVBucket will occur during a rebalance between the time the vbuckets move from being active on one node to another node and the time the configuration is updated. If the configuration is updated quickly (in less time than the operation timeout), the SDK should retry the operation and it should succeed. If the configuration is not updated quickly, the operation will fail.
During the rebalancing of Couchbase(Community Edition 6.5.1) cluster, .NET SDK v3.5.3 reported an error of “NotMyVBucket”.
Server 6.x is no longer supported, and improvements in configuration updates have been made since .NET SDK 3.5.3 (especially .NET SDK 3.6.1). Migrating to server 7.6 and .NET SDK 3.6.1 will handle rebalances much more smoothly.