Issue related with NameID.
Two things you can check:
- You created custom attribute but didn't 'make' it "nameID". [Here](https://www.gluu.org/docs/gluu-server/4.2/admin-guide/saml/#manual-configuration) is the doc.
- In your Dropbox metadata, metadata type is: `<NameIDFormat>urn:oasis:names:tc:SAML:2.0:nameid-format:emailAddress</NameIDFormat>`. So you have to prepare NameID in that way.