Thank you for your question. I'm sorry you're having trouble with the registration form. If the form is attempting to send out an email as well, it's possible that the new authentication settings could be preventing that and causing that error. Please make sure that you have authentication enabled in your settings if the form does indeed need to send an email.
Additionally, you may want to check subfolders of your public_html folder and make sure there are not other instances of hacked index pages in those subfolders. Quite a few customers have had index pages placed in folders that didn't even have index pages before.
Finally, removing and re-adding the plugin you use may help to reset it and get it working properly for you again.
I hope these suggestions help!
Question answered through second question here: Joomla registration form not working
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