iMovie - video rendering error?

I have the same problem just off the phone to AppleCare they tried to resolve the issue. By. making a short video then try sharing it which worked ok when I attempted to use my 55 min video it came up with ideo rendering problem error 50 they think the problem is in my video and asked me to delete it and start again I think the problem is. I movie it self I am now using the older version and starting again massive job to start again but I have no option

There may be more than one cause for this error, but in most cases the project to be published includes short clips of less than about 1.5 seconds duration.

Duplicate the project. From the duplicate, delete the short clips and try to publish. If that doesn't solve the problem, see the comments of Didorx in this thread and of jguttenberg in this thread.