6
2.0.4 is very old, contains bugs and many security issues. i would not recommend it. that and the fact that many newer modules will probably not work on it either.
the issues you may get would probably result in every1 telling you to upgrade to the latest XOOPS version in order to solve the problems..
think most module developers test on versions above 2.0.10, i don't think a high priority is given for older XOOPS versions. altho they do try for backward compatibility.