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You might try turning on debug and checking out error messages. If you don't understand the error messages post them here.
Some obvious suggestions are... firewalls and cookies. Often they cause problems.
You might also let us know which version of XOOPS you're running and provide a log to demonstrate the problem.
Having visited the site, you have the site loaded in a subdirectory not in public_html often this causes problems on a redirect.
Pax vobiscum,
...mark
may the road rise to meet your feet!
http://treemagic.org