As of right now CurseForge is down. Due to not being able to verify mod content PC players are unable to join modded servers. There is unfortunately nothing you can due to correct the issue. We have to wait for Studio Wildcard and CurseForge techs to correct the problem. On the bright side this issue is not affecting console players.
From WC:
SERVER
— Today at 9:11 PM
Hey folks, hope you had a good weekend! We're looking into an issue with our partners at Overwolf that is currently preventing players from joining modded servers.
@ARK News