Transaction Dispatcher has thrown an unexpected exception: Object Reference not set to an instance of an object

I have set up an IED REF615 and managed to connect to it on account of the green tick next to the IED in PCM600. I have also imported an SCD file to PCM600 and mapped the GOOSE inputs and configured the GOOSE publishing on the IED's side. I think everything is set up correctly but then when I try writing to the IED, I receive the error which I copied in the Title. From what I understood, this error is due to loss of connection but I don't understand since the connection seems to be stable.

The location of the error is at CRW.IEDFrontEnd.ReadWrite and the Exception Type is PCMToolException.

If you could please provide any guidance in solving this issue, I would greatly appreciate it. Thanks a lot.