Fawaz32: I ran in to this same issue downloading The Witcher 3, this is how i fixed it:
I clicked on my profile and went into account management, updated my birthday and it worked for some reason or another. Idk why this is even an issue
TheJigabachi: lmao, this is basically the equivalent of doing a raindance or praying or sneezing and expecting it to fix the issue. why would your birthday have any logical effect on this issue.
Good find, actually. The raindance may have an indirect effect which makes it become logical. If your profile data, stored on the GOG servers, has become corrupted or was stored in an old format, which the current version of Galaxy does not understand (properly), then, due to lazy error checking in the code, the error may silently propagate to the online check which just treats any error as "not online" instead of reporting the proper reason (this dumb practice is completely normal today, while it was common in the 19XXs to properly look for and diligently report any error; the current practice speeds up initial development time, but increases maintainence effort and bug fixing time).
Changing
anyting (birthday or else) in the account management updates the profile to the current format that Galaxy understands. That it only appeared after a reinstall of Windows may be due to cached data that were updated to match current Galaxy by successive updates on the client, but not on the servers.
This does not explain why the online installer fails, though, so there seems to be a piece missing in the puzzle, or two different issues that look the same thanks to the lack of proper error checking.
