Is it possible that this is with a long ago downloaded version? My bad for not properly tracking versions between different builds. Does it work if you re-download rpaex? [EDIT: nevermind, I'm seeing the error too now ......... and i'm investigationing .....]
I updated rpaex, now it should work on your game even if you encrypted/obfuscated the RPA files a little. [I don't know if it will work with all such games]
People who lost their sources and just need to recover some graphics/audio can also use DragonUnpacker.