Knowledge Doc TEC1831886 describes a problem which can cause EM communications to fail and includes a solution of increasing certain communications parameters to prevent the problem.
A potential issue with this is that it may be difficult to predict what value to set or if there is a particularly large amount of data involved, the memory requirements needed to accommodate it. So this memory requirement may not be realistic.
The main cause of the problem described in TEC1831886 is pre-8.x agents connecting to a MOM. These agents will not accept load balancing instructions from the MOM to connect to a collector and will also not adhere to the disallowed mode. So, it will try to send metric data direct to the MOM which will not be accepted.Thus blocking these agents via loadbalancing.xml rules will not work.
There is a hidden property which can be added to the IntroscopeEnterpiseManager.properties file.
isengard.server.banned.client.ipaddresses=<IPaddress separated by comma>
This is a static property -- it is only read on Enterprise Manager startup and requires IP addresses not DNS alias names
So, by adding the IP addresses of the rogue pre-8.x agents, you can prevent them from connecting to the MOM and causing the issue described in TEC1831886.