I've had the same error since the gmod update on June 25th. I can't get in the server anymore, and it's just the HZ PERP3.5 server. I can connect to EVERY other server but this one. I;ve already re-installed gmod completely, deleted my ClientRegistry.blob file, deleted my cache, changed my DirectX level, changed video settings, EVERYTHING. But, I still can;t manage to get in. Since it only happens when I try to join the PERP server, could it be an issue with the server?