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Well, because I made another installation of the same version of Xoops, with the same files, few days ago, I concluded the reason could not be a wrong definition inside the file.
You said the reason would be inside the file, then I found out that the problem was not the XOOPS core, but a corrupted file. In other words, it could be produced during the upload process.
Then, I deleted everything and made the upload again and... the site is working.
I must thank you, I would not think about this possibility without your comments.
Acris