Smannesman: Because they changed the account pages and that particular page no longer exists.
monkeydelarge: But why allow GOG downloader to continue pointing people to this page?
They're simply not updating it at all any more, instead putting focus on Galaxy. If the downloader continues to work - fine - but it's not being kept up to date with changes to the site. You will most likely be forced to switch eventually, either to Galaxy or to a third-party downloader if one gets built that suits you (a couple exist already: lgogdownloader for Linux is being kept up-to-date quite nicely, and jgogdownloader which is written in Java and should work for most systems but I don't know if that one still works).