You may notice that Service Gateway is sending information to a device other than the one that the information was meant for.
The ACS will attempt to identify a device through its IP if it receives an invalid "inform" package; it will also do so if the device does not put the cookies in its responses.
In this case, if IPs are unique in the system, this will not produce any issues. However, if more than one devices share the same IP address in the database, it could cause packages to be sent to the wrong device.
Giving the devices a unique IP, or configuring them so that the necessary cookies are included in its responses, should fix the issue.