Is this only happening with the migrated reports? Can you create a dummy report from scratch?
You might want to load the MR Configuration console (on the server) and make sure that your company has been set up under the ERP Integrations and that both the Application and Process services are running.
Getting MR up and running for the first time can be a bit complex. However, if you can create a new report and the error is only occurring with migrated reports then it sounds like the migration failed at some point. I know this is not a definitive answer to your issue but is a starting point.