I just recently began experiencing major problems with the downloader. It did have a habit of crashing randomly in the past (and failing to download files properly), but yesterday when I tried to download a bunch of games I'd just recently purchased in the sales, it refused to even connect to the server! I couldn't even log in to my account to begin downloads.
I tried connecting several times, restarting the downloader, restarting the computer, all to no avail. It just refuses to connect (and yes, my internet connection is fine).
Just an hour ago, it became worse. Now the downloader won't even open, it crashes before the login screen even appears.
I would open a support ticket about this, but the last time I had a problem and I tried to ask GOG for support, I received no answer (its been months now, and there still hasn't been an answer, luckily I managed to sort out my problem on my own, with help from the forums of course).
My only available course at this point is to uninstall the downloader, download it again, and reinstall. I can only keep my fingers crossed that this will work, as my browser's download speed is just abysmal.