If you have recently updated your store to Magento 2 version 2.4.2, this will cause your Shiptheory app to no longer be able to pull down order details, giving the below error:
Unable to download Shipment 1000018050 from Magento. Error 404 requesting...
To fix the issue, go into your Shiptheory settings and enter that your Magento 2 version is 2.4.2 and above.
If you have already implemented a workaround for this issue, please keep 'Earlier than 2.4.2' selected.
If you're interested in the background of the issue, Magento go into more detail here.
If you have any questions regarding this, please contact support.