On November 3rd at roughly 10:19am US Eastern Time, a code deploy caused a fatal exception for all Fandom and Gamepedia wikis on the new platform.
The error appeared as an "InvalidArgumentException" to users.
The offending code has been rolled back and the platform is back to normal, as of 10:40am US Eastern Time. If you see weird CSS issues, that's due to caching and should resolve on its own.
Sorry for the brief disruption!