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When I first upgraded from 2.03 to 2.05, I did a complete install instead of the patch. Plus I overwrote my mainfile.php. So when I did the install I had to put my database name, database username and password in again. I made a mistake and put in the wrong password. So when I finished installing the new version, I ended up with a blank home page.
It took me a little while to figure out that the reason was the wrong password for my database. So XOOPS couldn't connect to the database using the wrong password. As soon as I edited my mainfile.php with the correct password, my front page showed up.
Now your problem could be something little like this. XOOPS may not be able to access the database because you didn't put in the right username and/or password that you used to create the database in mysql.