Stuck on transmitting data

So sometimes when I invite my friend into my endless lobby or to my party for coop ranked, he gets stuck on the “Transmitting data” screen and after a minute it says “could not join lobby”. It happens when I try to join as well. This error started happening recently. Does anyone know the fix?

ALSO, I’m on the PC version and all of the ports are forwarded, NAT type OPEN and uPnP is enabled.