Hello-  I just upgraded today (to v1.5.207) and had the same situation (keeping old files and not uninstalling) and problems (missing media files message, etc.).

When I tried to import the big file (all 4 parts) it seemed to be working (shows progress bars, etc) but then instead of saying it was successful it says "Import was cancelled by user."

I tried again, rebooted, etc and tried the individual files (4 parts) which claim to work.  I tried with and without "Automatically resolve naming conflicts" checked and it doesn't complain except when I don't have it checked it wants to know what to do with duplicate categories (rename, skip, etc).

Anyway, I see 130 lessons.  If I try to play the first one it says "Media library files for Session 1 - A. Word Flash lesson not currently available."

I can also see the categories, but all the mp3 and jpg files just aren't in the Media Library directory.  Many are, but many aren't...  (the various 'clap' files for example didn't make it, but the 'cry' files were there).

The "Import was cancelled by user." only appears with the big file and not the individual ones.

I will try the fixes suggested and hope it works but maybe the info I have given here will help you to debug.