This error means that there is an issue with the synchronization between the ECDISes. There could be an error on the cable itself, or it could help to simply turn of all the ECDISes participating in the sync and then turn them on again. Note that all the systems must be off at the SAME time, before they can be turned on again.
Some times it might be necessary to reset the sync settings in the ECDIS service menu to get the synchronization working again.
We suggest that you try this first, and contact Furuno support directly if further assistance is required.