I already read the article (https://knowledge.safe.com/articles/886/working-with-geodatabase-subtypes-and-domains.html), but it doesn't seem to work. I get: "An error occurred while attempting to add the subtypes to the table/feature class 'Feature_Outline'. The error number from ArcObjects is: '-2147219885'. The error message from ArcObjects is: {An expected Field was not found or could not be retrieved properly."