Indexes are 0-based in PAD. So, if you have only renamed a single file, you should use %RenamedFiles[0]% as the file to launch.
Also, you should make sure that your Rename file(s) action actually renames the file. Make sure you set it to "Overwrite" if the file exists already.

By default, this is set to "Do nothing", meaning that if a file with the same name already exists, the target file will not be renamed. When that happens, %RenamedFiles% will be empty.
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