com.couchbase.client.core.state.NotConnectedException

recent 2 days we met the com.couchbase.client.core.state.NotConnectedException error in our Prod server. never has it before.
any idea what’s the possible reason for this error?

below is the stack trace

org.springframework.dao.DataAccessResourceFailureException: nested exception is com.couchbase.client.core.state.NotConnectedException
at org.springframework.data.couchbase.core.CouchbaseExceptionTranslator.translateExceptionIfPossible(CouchbaseExceptionTranslator.java:87) ~[spring-data-couchbase-3.1.5.RELEASE.jar!/:3.1.5.RELEASE]
at org.springframework.data.couchbase.core.CouchbaseTemplate.execute(CouchbaseTemplate.java:555) ~[spring-data-couchbase-3.1.5.RELEASE.jar!/:3.1.5.RELEASE]
at org.springframework.data.couchbase.core.CouchbaseTemplate.queryN1QL(CouchbaseTemplate.java:509) ~[spring-data-couchbase-3.1.5.RELEASE.jar!/:3.1.5.RELEASE]
at com.******.getFirmAccounts(PositionsAccountQuery.java:179) ~[classes!/:na]

PositionsAccountQuery.java:179 results.addAll(this.query.queryN1QL(apacX).allRows());

Likely due to a network connectivity issue.