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I had a similar problem. Yours may be file related, I have searched the forums and no answer yet that I can see.
I sorted my problem by going to my XOOPS database and the xoops_users table using phpadmin.
I discovered when clicking on the table and going to "browse", that the last user had an id of "0". After clicking on "structure" I discovered that the "uid" field was not set to "auto_increment". I set this and deleted the user/users with a uid of "0", this fixed my problem.
Long shot, it may not be this in your case, but you could go to the "sql" file in your XOOPS "install" folder in your download and check that all the field properties match.