Sachys: dont know about gog but the steam version has received a large update and multiple smaller patches recently. if gog is remotely on par then that could be the cause and youll just have to await another update / patch.
Edit:
just loaded it up on steam. the splash screen says v1.05 - i doubt the patches have been included in that version number but check against your own and see.
also, in case that turns out to be useless i dont have that dlc, just guessing that may the root issue.
I don't intend to wait that long to backup my updated game. zowie just forwarded my support request so they should resolve it pretty soon.
Otherwise, I know exactly what the root cause of the problem is: Once in a while when they update games, something funky happens that results in a dead link in the browser. I don't know what breaks in their implementation exactly, but I believe the remediation is for them to upload it again (or in any case reset something on their end).
Given the relatively low number of users that don't use Galaxy, have that dlc and want to download the Linux version, this problem could have gone unnoticed for some time (my guess would be days) before someone reported it.