This tutorial shows that how to fix " Cities: Skylines Afterdark DLC " Loading Screen Stuck Error.
You need just simple steps to do it.
Disable All Mods
Run your game through steam with "-disableMods" and "-noWorkshop" .
You need to disable all of your mods* Then this will work.
Uninstall / Reinstall
Unistall the Cities Skylines
Run your steam as an administrator.
This hotfix works great for this fix.
For more detailed video instructions you need to have premium memberships.