Manager Server - Invalid Dispatch Identification error

When trying to configure a connection from the Manager Server to a remote server,I get the following message:

java.lang.IllegalArgumentException: Invalid dispatch identification

I can do an http post to the remote server from teh Manager Server, using the same port, user and password, so basic connectivity is there.

The manager server is running under IS 4.6 with Manager server updates for 5.0.3. The remote server also has 5.0.3 updates.

Any thoughts on what the problem is?