You may get the error when you try to check in or add files.
Cause & Resolution
It might be the firewall issue. Please make sure that the port (80 or 443) and the IPs (72.15.153.*) are not blocked by your PC firewall, router firewall, proxy agent, and etc.
Unstable network can cause the issue too. Please check the "Check if the socket is valid before network operations" option in menu Tools| Options| General tab to see if it helps.