Hi John,
Can you share the name of the file that you are trying to use?
The GUI "manage patch files" might not successfully work depending on what update file you are using.
The recommended way for upgrading from 2.4.x -> 2.5.2.1 is via the command line (admin shell), using the commands:
#check update
#download <patch name>
#apply <patch name>
Regards,
Cesar