I've been trying Retrospect 10 for Mac for a few days and I've had lots of problems with a Windows client. Error -519 almost instantly (dropped connection). Searching for the error online gave me no results. I've tried everything (updating drivers, checking for network problems), but no result. The last thing I tried was uninstalling Avast! completely and my problems were gone. Note that disabling the shields isn't enough, mb the local proxy is always active? So, if you have -519 errors, check your antivirus software, especially with network shields etc. It might save you some time I replaced Avast with Avira and everything is running smooth now!