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Could be a permission problem. Check that the appropriate folders have read and write (777) permissions. Off the top of my head the templates_c folder is one. Check the instructions - there could well be others.
Also, check that your borwser allows cookies - XOOPS is heavily cookie dependant.
Lastly, check your firewall. If your firewall is blocking HTTP URL Redirection that may be causing the problem (your browser doesn't know what page it just left so it loses the values). Do a search for 'HTTP_REFERRER' and you will find a post that I and many others have posted in about disabling this check - its a simple bit of code adjustment.
If none of that works, re-post but give more info about your server configuration. I am asuming you are Red Hat (RH?) but others may need to know more (I use Windows Server 2003 and IIS6 so am unlikely to be able to help with that bit!)
Good luck