Some ideas
ifg you use the java sun engine try to enable the java console and look there for any errors.,
second hint, look at the source code of the wiki3d popup window here and on your site, any differences that can cause this error?
third hint editing the template file tiki-wiki3d.tpl via the tikiwiki frontend is maybe not a good idea. I did this and the applet codebase tag was removed while saving. Perhaps there is some code cleanup while saving templates. After restoring this file from the original sources evrything was fine.
Greetings
Andreas
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