Unable to recognize document as valid XSD. Check the selected category

Hi,

While loading the attached XSD using "External Definition" I am getting the error "Unable to convert imported document to WSDL Reason: Unable to recognize document as valid XSDCheck the selected category".

Does anyone know how I can fix this problem? I also tried to delete the minoccurs = "0" because I have seen in other threads that this might solve the problem but it did not help.

Thank you very much for your help!

Kind Regards,

Christian