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Thank you MadFish & xuser. I reinstall the os for increase the partition of /var. Before reinstall, I login as root and use the K3b to backup the xoops. After reinstall, I login as root and use copy & paste from CDrom. There is nothing strange during this process.
The pages before login is quite normal. When I try to login, the wellcome page show (wellcome to login). Right after that, I was directive to first page again. So, I can reach the administration page. It is so peaceful, XOOPS did not tell me anything wrong.
Really need help. Thank a lot.