jsonrpc error in Claymore: ProvidedPoWsolutionisin valid

Hi all,

Sometimes I get the following error while the rig is mining:
ETH: Received error: {"jsonrpc":"2.0","id":4,"error":{"cod e":-32700,"message":"ProvidedPoWsolutionisin valid!"}}

As a result, Claymore 9.2 shows the usual hashrate, but the pools says it has stopped. Actually the machine is mining for nothing.

What I found is the description of the 32700 code: Invalid JSON was received by the server. An error occurred on the server while parsing the JSON text.

Any idea what can cause this error? Is it the pool, or the internet connection, or something related to the machine itself?

The rig is 6x470 Nitro+ 8GB

Thanks!