The error (An Error Occurred when attempting to: When Creating DeutschePost Shipment! API Error: Error:The remote server returned an error: (500) Internal Server Error.) from Deutsche Post can be caused by the LabelType being incorrect, ZPL is the most usual format to use. To check this please do the following:
Click Connect then Courier Integrations.
Next click Deutsche Post.
Find the account and click Edit.
Change the LabelType to ZPL.
Then Update.