We need to configure and create Couchbase Client in runtime. I create new CouchbaseClientConfiguration :
var config = new CouchbaseClientConfiguration();
config.Bucket = _couchbaseClientSettings.Bucket;
config.Username = _couchbaseClientSettings.Username;
config.Password = _couchbaseClientSettings.Password;
var client = new CouchbaseClient(config);
But this client is unable to find another couchbase node in cluster if the node it was pointed to in configuration file is down.
When I create Couchbase client with CouchbaseClientSection from Web.config there is no such problem.
I was able to resolve the issue by explicitly creating NodeLocatorFactory in code:
config.NodeLocatorFactory = new KetamaNodeLocatorFactory();
And after that it works fine.
But it is still magic and seems like a bug.