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So I looked at my last cms install and noticed a difference in mainfile names.
The previous installed cms had the file named "mainfile.php".
The 2.5.9 install has the file named as "mainfile.dist.php".
Renaming the file to "mainfile.php" throws a server error on access....
I recall some subtle thing about this, just not exactly what.
(I will mention here that the installer guide has 0 info about editing this file before install. Which makes those instructions useless to newcomers.)