I had no problems transferring, and I've also sold a few paperbacks since with no negative reports. I probably should have ordered author copies of all of them but haven't so far. However, I've had a new release since then, did order a copy of that, and it came out fine.
Kay, the slow internet could be an issue, but if that were the only problem, the transfer should fail with some kind of error message, not just freeze. (I think that's the case if something times out, anyway.)
I don't recall if you mentioned other troubleshooting steps, but I find it can be helpful to try different browsers. For instance, I can't do the online routine with Chase to verify receipt of a new credit card if I use Chrome, but it goes through fine in IE. Over the years, I've seen that happen a lot of times.
How you access your service can also be an issue. A while ago, I switched to a VPN for privacy reasons, and it works fine for most websites, but there are a few (Barnes and Noble, oddly enough, is one of them) that choke when I tried to browse using a VPN. I have to turn it off in those cases. Of course, if you're accessing the internet without the intervention of a VPN or other intermediary, that shouldn't be an issue.