Does this happen every time you try to connect to the server?
Try the usual basic fixes like:
-Restarting your modem/router
-Restarting Steam Client
-Switching your vpn on/off (if you have one)
#Disconnected by S2C_CONNREJECT (Bad Challenge)
rustic oxide
solemn frigate
Does this happen *every time* you try to connect to the server?
Try the usual ba...
Yeah, every time. I tried the above and reinstalled gmod itself, nothing worked.
rustic oxide
If it only happens on this one server, and nowhere else, it implies the issue is on their end. Could just be a temporary issue too if the connection between you and this server has packet loss ("hiccups"), so try connecting again later.
Are you able to contact the admin about it, maybe to give the server a quick restart just in case?
And you may have checked this already, but have you made sure Gmod&Steam are both allowed through your firewall/security apps?