Firek: Using the Downloader is the, arguably, most convenient workaround, but here's how it works:
Our browser download links are (among other things) IP-sensitive, to accommodate browser download resuming while remaining secure.
This works under normal circumstances, but, unfortunately, we have learned that, in very rare cases, if a user's connection assigns a different IP when connecting to our site, and then a different IP when the actual download is initiated with our file hosting provider, an IP mismatch occurs, resulting in the 403 error. We are currently unable to resolve this issue, whilst preserving our users' ability to resume browser downloads.
If you've set up a proxy connection yourself, you should make sure that the same proxy is used for all connections, rather than only some of them.
If all of this happens in your ISP's back-end, it may be possible for your network administrator to modify your network setup so that you are issued the same IPs for both GOG.com, and our file hosting provider's servers, which you eventually get redirected to. Unfortunately I cannot guarantee if this will work, nor if the network administrator would agree to making such a change.
The IP mismatch is something I have gotten used to, being forced to use a proxy for so long. However, most downloads are not a problem
In fact even GOG was not a problem, until recently.
Unfortunately, I do not think I can get my ISP to do anything at all.
The reason I am posting this is because I expect GOG to be the one to do something about it. This 403 error shouldn't be happening at all. This is a problem on YOUR end, rather than mine.
I have downloaded from various websites without problems, such as uploaded.to, and mediafire.
The reason why I came to GOG, is because I don't have to deal with crappy DRM, forced online authentication or constant internet connection.
Now if I'm going to have a problem downloading from the actual servers, I might as well be pirating the games off bittorrent in the first place.