Hi Nicolas!
Here is my problem:
I tried moving this Joomla 3.3.6 site -- which was working just fine -- to the root directory from a subdirectory and as soon as the backup files are unzipped in the root directory - before running the installation wizard - I get a 500 error--can't access the installer, or anything.
I thought it might be something weird with the server so I also downloaded the backup file and uploaded it to a different server (but same Host) and had the same problem.
I don't know what to do to fix this! I hope you can help me.