unable to prepare project for publishing error (-50)

Hi,

After adding a slow motion effect to one clip of the project, iMovie started to fail to preview the images when moving the mouse over clips (in the storyboard), then a few of the clips just wanished from the storyboard and became grey.

Project can't be previewed using the play button.

And I get the error (-50) pops up when trying to finalize the project.

Any idea how to fix this ?

Thanks.