Hi, Estone70,
It could be that there is some corruption in the project at the 6 second mark. Or possibly there is a corrupt preference in iMovie. Also, You Tube has a duration limitation of 15 minutes or so (or at least they used to have one) and you needed to sign in and register to post longer videos. So you might check You Tubes rules.
Try exporting to your desktop and see if you get a complete export. If not, scroll through the first few seconds of your project and look for black frames, white flashes, artifacts and any other signs of corruption. Cut out any that you find and try again to upload to You Tube. Most reliable way is to drag your desktop video into You Tube's upload window.
If you don't find any signs of corruption, try deleting preferences. Deleting preferences is a safe procedure that will not cause data loss or disruption to your project. To delete preferences open iMovie while holding down the Option and Command keys and select to delete preferences in the box that appears. iMovie will open in a new library. Reopen your old library to get back to your projects. Now see if you can get a complete export and/or upload to You Tube.
-- Rich