Bad challenge


New Here
Hello, I'm playing css with 3 other people in a lan. Normally we have no internet but lately I did an update because I shared my phone's internet with my computer and the other ones in the network, so they all have the latest update.

But now, when we want to play, some have problems to enter the server/game. The loading screen is there for a long time and after that there's a message 'bad challenge'.

Now when I enable my phone's internet. Then they can enter the server without a problem. The problem is that I'm not able to always
enable my phone's internet. We still use some old computers with windows xp sp3.

How can I fix this or do I have to downgrade the game ?