This error has been the most troublesome for me in the past. Typically of you try searching the error you will get pointed to KB article 943948. What I have found is that this is caused when you run an integration and Microsoft Dynamic GP seems to disappear and Integration Manager freezes or gets closed down integration. I have also seen this occur after an error and the user tries to re-run the integration.
What is occurring is that Integration manager and Microsoft Dynamic GP still has processes running in the background. If you go into task manager and look at the running processes you will see multiple for Microsoft Dynamics GP for the user trying to run Integration Manager. You have two solutions, either go into task manager to stop the processes and test again or have the user reboot.
Sometimes just stopping the services does not resolve the error and a reboot is still required. If a user is logged into a terminal server it is important to verify that they fully log off the server and simply does not disconnect.