I assumed it to be firewall issue as well, but this doesn't seem to be the case since I disabled and even uninstalled my fw temporarily for testing purposes. However, I figure it has something to do with the server certificates. When I use the launcher and "check certs" it will tell me that the certificate is not installed and whether I would like to have it installed. When I continue it will randomly either do obviously nothing or pop a warnig that it could not receive the certs from the server since it wasn't responding.
A friend os mine has EXACTLY the same issues although he is using Windows XP Home 32bit and ZoneAlarm fw and I am using Win7 64bit and Comodo fw, so it can't neither be a UAC or fw problem afaik.
I'd append an error log but strangely the launcher doesn't create one.