X-Rechnung / Peppol generation fails: "Null file path not allowed - Skipping XML Export"

HI there,

I am trying to use the X Rechnung / Peppol functionality of B1 FP2018, but the generation in the electronic documents folder always fails with error "Null file path not allowed - Skipping XML Export". I double and triplechecked the path where the xml file should be placed, everything is correct, but it still doesn't work. Any ideas what could be the issue here?