Usually, sensitive data (like login and password) is encrypted and it's encrypted version saved in database. Take a look on this example (it shows md5 checksum of words "Lhademmor@" and "lhademmor@"):
As you can see, just one difference ("L" and "l") changes the whole checksum - thus, you couldn't login.
I tried both. No luck. However, my password contains both capital and lower letters, but surely that can't be an issue? As I said, it works on GOG.com...
Have you tried changing your password? Since when does the problem occur?
I just tried changing my password - was able to log onto GOG.com with the new password, but still no luck with the Downloader.
And I only decided to "GOG up" my MacBook Pro today, so this is the first time I'm experiencing the problem.