I have tried printing a Royal Mail shipping label and am seeing an error returned:
'Unable to provide the label requested
Reason: Error E1356: An unknown API error has occurred. Please contact support.'
The error is returned due tracking ranges running out on the service being used on the order. An automatic request is made to obtain new tracking ranges when they are nearly finished, so if the above error is returned, it means that new ranges have not been allocated yet by Royal Mail API. Please allow some time for the request to get processed and try printing again.
If the error is returned for over an hour, please contact Linnworks support.