The interoperability of XMI between tools was always an issue. OMG tried to address this with tighter rules for the 'Canonical XMI'.
Many UML tools are now able to import models expressed in Canonical XMI. It would be great if Staruml could do this as well.
Canonical XMI is described in the Annex B of the documentation of 'XML Metadata Interchange" at https://www.omg.org/spec/XMI/.

Furthermore, it would make sense to update the namespaces of UML and XMI to more recent acknowledged versions.

An ideal solution would be that the XMI export of Staruml also follows the rules of Canonical XMI.
This way models created in Staruml could be imported into other UML tools without issues.

The DDI-CDI UML class model expressed in Canonical XMI could be a test case for this.
See: https://bitbucket.org/ddi-alliance/ddi-cdi/src/master/build/encoding/xmi/ddi-cdi_canonical-unique-names.xmi