A user reported that LADkit failed to install with Apache mod_security enabled. It's possible that mod_security objects to the base64 CSS image data built into Simple PHP file manager.
The problem was solved by temporarily disabling mod_security.
WARNING: mod_security is powerful but difficult to handle. Without very careful configuration, it is likely to cause unexpected errors. Often, these are not visible - things simply do not work. Unless the person managing mod_security is able to apply significant effort to configuration, it is best not to use mod_security.