I constantly had this until I applied the following fix. Other issues included the ability to see other matches but not join. My machine is running Windows 8.1 x64 so there may be differences in your paths. For me, these errors occurred because although the installer added the AVP executable to the Windows firewall, it didn't add the GOG client itself which would seem to be important as it's running through that to connect multiplayer matches. Add the exception to the Windows Firewall by opening Control Panel>Windows Firewall>Allow an app or feature through Windows Firewall>Change Settings>Allow another app... and browse to "C:\Program Files (x86)\GalaxyClient\GalaxyClient.exe".