@weber
Sorry, to disagree with Diva just a little bit, but you can get the installer to run again just by re-uploading the includes/auto_loaders/config.image_handler.php file.
This file is deleted after a successful install. Which stops the installer running again and again and again. It is this file that triggers the installation process. (Which is why if you get into problems with the installer it is the one to delete to stop the process)
If you re-upload it and all the other files are in the right place then the installer will run again. It will overwrite the core files again and create new back-ups.
BUT
what Diva says is still the best way of proceeding. Upload the whole thing again and off you go.
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