I've tested it some more and I still think it's a problem with the googlecheckout.php file located in \includes\modules\payment\ folder.

Don't forget, there's 2 files named googlecheckout.php with completely different content. The other is at \includes\languages\english\modules\payment\ and they're extremely easy to mix up. I would upload all the installation files again, one by one, and test the results. I'd also check the settings on the FTP client just in case the files are getting corrupted during the upload.