The error is caused because your certificate has expired. The Hyperledger Fabric caertificate for version 0.6.1 has a validity of 90 days. The certificate that you have is valid only for that length of time. You can read more about this here: https://jira.hyperledger.org/browse/FAB-1996
When using Hyperledger Fabric version 0.6.1, the administrator of the blockchain network is responsible for the renewal of certificates.
During the certiicate renewal, the chaincode is stopped. To start the chaincode again, you have to run a query or invoke the chaincode from all the peers.