I've mentioned this a long time ago here, and this ridiculous bug still persists.
Basically, when you go from Final Cut Pro (6.0.5) to Motion (3.0.2) and then back again (via saving in Motion) sometimes the .motn file that is initially placed in the timeline will be completely black. Also, sometimes, when you open it in the Viewer, it's fine; putting it manually into the timeline it looks fine but unrendered. Render it and it's all black again.
Bizzarely, create a new sequence and place the .motn file into the timeline and it'll look fine, render it, and it'll be black.
Worst of all, export from Motion and the resultant Quicktime file will sometimes be all black.
There's another thread on this here.
Stranger still, in Motion, Export Selection... will do nothing when In and Out points have been set.
This must be some kind of rare bug (possibly confused alpha channels or frame rate mis-match) or settings issue with Motion or Final Cut that spans software versions, years (yes, years) and Mac models and graphics cards. I'm experiencing this issue on not only with a newer version of FCP than last time, but Motion as well, as well as a model of Mac that wasn't available at that time with a graphics card unavailable at that time.
The solution? For me at least, there is no solution, only a workaround of exporting out of Motion as an 8-bit Uncompressed file then import that into Final Cut Pro.
If you have any information, tips, hints or anything about this please let me know. I'm not the only one dealing with this issue judging from a few people I've asked about it.
Here are some links to threads about this issue (and similar ones) from some emails I've received:
FCP interprets .ipr & .motn files (correctly) with Black alpha. Since when?
Livetype problems since going to FCP 6.0.3 and QT 7.4.5
Livetype problems with K3
Live Type maybe fixed?
Livetype Effects misbehavin'
Suggested Possible Solutions:
01. Change the Alpha of the Motion files from straight to black (or vice-versa).
02. Go to the View menu and change the preset for playback to none or at least something other than what it is set to now.