If a customer is unable to upload an ETD letterhead or signature image but their credentials are working and they are able to book shipments OK, check when their FedEx integration was activated/added to their Shiptheory account.
Before April 2019
If the integration was added on or before April 2019 then the issue is likely that their credentials don't authorise access to the FedEx UploadImages API causing this problem. In this instance, the customer will need to remove their integration and re-add it by completing the "new" registration step to re-generate their credentials.
After April 2019, the updated FedEx integration automates customer registration and generation of credentials for all of the FedEx APIs Shiptheory uses including shipping and uploading images.
After April 2019
If their integration was added after this date, it's worth removing the integration and re-adding it to generate new credentials to see if this resolves the issue. According to FedEx EMEA WebServices support, this happens extremely rarely.
If the problem persists, please contact FedEx EMEA WebServices support (firstname.lastname@example.org) to investigate further.