Hello !

Since last Wednesday, when I try to wishlist one game (with Galaxy), it asks me my password, but I am already logged in...
If I enter my password, I get an error 404 page...

Is it a bug ? Anyone knows how to fix it ?

There is a current bug with wishlisting games using Galaxy. Until it is fixed, there is a workaround on this thread: 
https://www.gog.com/forum/general/cannot_wishlist_games_anymore 

Or you can just log in with a regular Internet browser and wishlist the games there.

Probably just a bad Galaxy cache. I can't remember specifically the instructions and paths to help you refresh it manually though.
If it keeps up and it's a problem just run it by support and they should be able to tell you how.

EDIT: Never mind then ;)

Thanks guys for your replies, the solution worked fine for me :)