If you've been having problems logging into Horizons today, the developers might just have the reason. It seems that along with their maintenance, they added on a security update that could be causing issues for some players.
Attention Players: During the maintenance today, we made some updates to our security system. If you are one of our players who use the standalone launcher (HorizonsLauncher.exe), you will need to patch through the website to get the updated version before it will work properly.
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