FIXED - You should now be able to install/update without glitches again via GOG Galaxy :) At the moment, installing/updating games may give you the "NOT AUTHORISED" error message. We're investigating the issue.
Moved this comment to just below the original post, as I will be updating the status of this issue when we have more info :)
UPDATES: -- [15:35 CET] Issue fixed! You should now be able to download games and update files smoothly :)
-- [15:11 CET] We've located the problem, and are working on it now.
-- [15:08 CET] The issue appears in
GOG Galaxy only - so you'll be fine installing your game or updates manually at the moment (GOG Galaxy will recognise them as installed later).
-- [15:05 CET] The issue
does not affect all users or all attempts at an installation or update (some users report it working on their 2nd or 3rd try), so you can make multiple attempts.
IN GENERAL, THOUGH:ElTerprise: Are you referring to Witcher 3? Have you 'bought' the free DLC Program? Because if not that can give the Not authorised message.
From the FAQ:
Q.: Upon installing the retail copy and redeeming the code on the website, the game fails to update through Galaxy with the status set to NOT AUTHORIZED.
A.: You need to 'purchase' the free DLC program first, once that is installed it should automatically go on with the updating process.