4
I tried with Chrome and didn't have any problems.
This is just a wild guess, but maybe it is somehow related to browser's cache and cookies, where they first check, and because you have "registered" user cache, then it generates conflict with the "anonymous" ?
Again, I have no idea, just a guess...
Try to clear your XOOPS cache, and the cache in Chrome and and Firefox and then try again as "anonymous" and see if there is a difference...