The problem is like something we've helped with several times in the past, mostly in the Three Big Upgrades (1.0 to 1.5, 1.5 to 1.6/1.7/2.5 and 2.5 to 3.x). When your backup is a different version family than the currently installed version you cannot restore the backup directly on top of your site. For example, you cannot restore a Joomla! 2.5 backup on top of a Joomla! 3.x site. You can neither restore a Joomla! 3.x backup on top of a Joomla! 2.5 site. This is a corollary of how Joomla! detects and loads its core files. Mixing files from different version families causes the whole thing to come crashing down in flames.
The solution is a leap of faith of sorts. Make sure you have kept several copies of your backups before doing anything! Then you need to delete all your site's files and folders. Now you may restore the backup and it will work just fine.
Nicholas K. Dionysopoulos
Lead Developer and Director
🇬🇷Greek: native 🇬🇧English: excellent 🇫🇷French: basic • 🕐 My time zone is Europe / Athens
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