A network-related or instance-specific error occurred while establishing a connection to SQL Server 2016

My SQL Server is 2016 SP1, Enterprise edition, installed on Windows server 2016 Datacenter.

It's always-on cluster, master and standby. in sync mode.

I am getting from time to time the error:

ERROR 2019-12-25 07:28:48,523 class:ODBCWrapper.StoredProcedure topic:null method:ExecuteDataSet server:IP-AC1F6CDD ip:null reqid:null partner:null action:null uid:0 msg:While running : 'Get_customerID' Exception occurred: A network-related or instance-specific error occurred while establishing a connection to SQL Server. The server was not found or was not accessible. Verify that the instance name is correct and that SQL Server is configured to allow remote connections. (provider: TCP Provider, error: 0 - The remote computer refused the network connection.) System.Data.SqlClient.SqlException (0x80131904): A network-related or instance-specific error occurred while establishing a connection to SQL Server. The server was not found or was not accessible. Verify that the instance name is correct and that SQL Server is configured to allow remote

Is it a network issue, or a database issue?

Can someone give me a hint (or an all solution :) ).

Thanks