Hey Paul,
The only things I can think of that would prevent the files from being altered is either the folder permissions aren't correct, or the files are not located where they need to be. I have installed this product a multitude of times on many sites and never had any issues. However, none of these sites had the "admincp" folder renamed.
The install code is PHP, and I have used it on sites running PHP 5.2.x - 7.x.
Just to double check, before you install:
- Did you create a folder called "admincp" in the same folder your renamed "admincp" folder is located?
- Did you put a copy of your "ranks.php" file there?
- Ensure permissions of "admincp" folder is 0755 and permissions of "admincp/ranks.php" is 0644?
- After install, copy the files (the backup and the altered "ranks.php" file) in the "admincp" folder to your renamed "admincp" folder?