Every time I launch galaxy I get an update failed error (E6) for Asterigos: Curse of the Stars

Sounds like a case for a support ticket.

Is the game listed in your library on the website?

yep

It's weird but they seem to put more bugs into every new version of Galaxy. I still don't have the "Extras" button in Trine 5 so I can only download the game via the website.
I know some users who are using an older version of the client and they can download Trine 5 with no problems.

I only install my games from downloading from website. I only use galaxy for updates.

I use Galaxy to download my installers and to install a game I use those. This way I can see if the installers are okay.

I do it that way because that's the way I've always done it since 2015 and Galaxy didn't exist back then. I hate running launchers. (Uplay, EA, Rockstar, Steam, Epic, Battlenet ect)
I keep all my games on a removable hard drive for safe keeping.

I only use Galaxy. It offers a great way to keep overview over my games, like I can sort them by "last played" and stuff :) Same here except it's a NAS plus backup HDDs.

Happens to me often. When it pops up, go to Task Manager, kill all Galaxy processes and launch Galaxy again. Usually works the 2nd or 3rd time.

I'm stuck on an outdated version and and get this error every time I launch it after completely quitting it before.