Hallo!
I tried to open a poseidon project in argouml 0.22 and have many problems:
I managed it to load the file in ArgoUML, when I delete all tags with UML2 in the xmi file and turn on the user preference to ignore Diagram elements (in user preference Settings dialog - as explained in the FAQ).
T. Weise, M. Zapf, M. Khan, and K. Geihs. Proceedings of the 7th International Conference on Hybrid Intelligent Systems (HIS'07), page 332--335. IEEE Computer Society: Piscataway, NJ, USA, (2007)
T. Weise, M. Zapf, M. Khan, and K. Geihs. Kasseler Informatikschriften (KIS), 2007, 2. University of Kassel, Fachbereich 16: Elektrotechnik/Informatik: Kassel, Hesse, Germany, (2007)
D. Frankel, P. Hayes, E. Kendall, and D. McGuinness. 1st International Workshop on the Model-Driven Semantic Web (MDSW2004), Monterey, California, USA, (September 2004)
J. Pleumann, and S. Haustein. UML 2003 - The Unified Modeling Language. Model Languages and Applications. 6th International Conference, San Francisco, CA, USA, October 2003, Proceedings, volume 2863 of LNCS, page 190--204. Springer, (2003)