My friend and I were having the same problem and I was able to fix it by modifying some server settings.

[servertest.ini]
'PingFrequency' The description is "time between each client ping, in seconds" I changed ours from "10" which is the default to "5"

'PingLimit' The description is "Ping limit in milliseconds before being kicked from the server' Default is "400" I changed this value to "1000" alternatively one could set this to 0 to disable this entirely

'CoopMasterPingTimeout' I could not find a description for this setting, but following the theme since it seemingly deals with timeout I doubled it from '60' default to '120'

The result of these changes is that my friend and I went from crashing the main menu all the time to where it was almost unplayable to never crashing once after these settings were modified.