Allmost all clients on my server take this error and disconnect:
Invalid challenge packet.
Responder : [IP-ADDRESS]
Protocol : 255
AppID : 0
Players : 0
MaxPlayers : 0
Bots : 0
Server Type : ???
OS Type : ???
Password : no
Secure : no
Your connection to the server has timed out.
after a while i had this error too, i tried to join other community server and a valve original server but they all give this error and timed me out, this must be resolved, there is not problem in my router or my firewall or my anti-virus or my modem or my isp it's a general problem not custom one so please fix this issue.
And here's a tip, after the client change him client port there isn't problem any more, but if client has 27005 client port then the client getting this error and disconnection from the server.
I have a cs:go server host and i have over 100 server and they all has the same issue, i tried to fix this on incoming/outgoing packets but the packet that has tis challange response is encrypted(i guess) cause i didn't find any packet that has ffffffff41 content.
this to packet doesn't send from the server side, cause there isn't any packet like this content on server side or packets are encrypyted.
I tried to send my client with my server ip adress to an invalid packet that has ffffffff41 content, when client is in game, after sending packet client console said invalid challange packet, so i think the fix is disabling disconnect from game when client gets an invalid challange packet.
sometimes this happens on valve servers too, this has to be fixed as soon as possible, it is an urgent problem.