Suddenly out of the blue Gog Galaxy not logging in.

Oops something went wrong.

Turn off Router for ten minutes. IP blocking.

Disable IPv6 and restart your system.

If that doesn't help,
• Double-check that the entirety of GOG Galaxy isn't blocked in your firewall. There are multiple executables under C:\Program Files (x86)\GOG Galaxy and C:\ProgramData\GOG.com\Galaxy\redists
• Start Reliability Monitor and check if any of the GOG Galaxy processes are crashing in the background.

Throw Galaxy out the window, and just use offline installers. If you want a management system, you could try Heroic, Lutris, or whatever the heck it is they have for Windows these days.

Disabling IpV6 is just gonna clog your network.