Log-in Timed Out when online mode=true

I have a bungeecord server running (version 1..7.9 #974).

When I put "online mode=false" in the bungeecord config file, players can log in without any issues.
However when I put "online mode=true" in the bungeecord config file, players get a "Timed out" error.
Are there any other settings i have to change when using "online mode=true"?
If I understand it correctly, "online mode=true" should only allow premium minecraft players.

Thanks for helping me out,

Rob