I happend to be in your same shoes this is what i did. Restored DB backup and files back to 3.4 (i was on 4.0.11) Then set the IPS as default theme. (Moved all files with ftp to a folder named _348, just so i have a backup) Uploaded all IPS 4 files. Ran the ugprade, BUT. I only selected the following to upgrade. ForumCalendarSystemThe shoutbox and chatbox i did not upgrade. Once updated i installed them after i had access to my IPB panel. Might not be the answer but i failed the upgrade 4 times, because i was to fast until i did it that way.