What does it mean ERROR code=-28, message=Activating best chain?
Your node is still starting up, you need to wait until it is finished before using the RPC interface. The RPC message is telling you to call back later, which insight-api doesn't seem to be able to understand, simply waiting until the node is completely active is a suitable workaround. This may take a few minutes depending on your servers hardware, the bitcoin-cli tool supports automatically waiting for the interface to be available with the -rpcwait flag.