Importing Video from Canon SD400 "Unknown Error"

welcome Ketene to the  board 🙂

first of all:
iMovie is a video edit app meant to work with firewire connected miniDV camcorders

.avi from a still-cam...?
needs conversion.
I don't know, what exactly is inside that .avi (.avi is just a container, able to contain many formats), but I'm very sure, it is an mpeg, try this free tool:
Streamclip

that will convert the content of that file into something useful for iM.

reading iLife06:
have you tried to import the footage in iPhoto6?
easier workflow:
connect camera, launch iPhoto, import "videos" (iPhoto does the conversion), export to iM......

for work with iM you allways should select "dv" or "dv stream"…

that is the native, internally used "format"/codec, which is the same as on miniDV tape.

Have you checked your root level library in the QuickTime folder to see if there are any QuickTime plugins that might be conflicting with iMovie 6 since you upgraded from a previous versio? Check out /Library/QuickTime and see if there is anything in there that says DIVX or Flip4mac. I have seen files in that folder cause problems on my computer when I upgraded to iLife '06 from previous versions when I had stuff installed that was compatible with the earlier version but no longer worked with the new version.

You may want to check out your user Library folder as well and see if there is a QuickTime folder in there. Anything in that folder should be non-apple so you may want to try removing them as well to see if you can import the files.

Mac Mini Mac OS X (10.4.6)

The suggestion of removing the components worked for me. Thanks.