The JMS server is running on a weblogic 9.2 cluster and on the same weblogic servers as the Business Service. This problem didn't happen when the same application (service + jms) ran on a single machine.
The strange thing is that this doesn't happen all the time. The last thing that i did to "fix" the problem was to update the passwords for the users used in the different services , to the same password as before i might add. I then used a user/password to access the Proxy Service and then it started to work again after that. 2 days later it stopped working again, "Invalid Subject".