Updating to itunes plus
Please try to turn off this kind of program then try to convert again.
Or you can also launch and use program in safe mode.
Before converting, make sure your Mac / Windows computer is well authorized with the same Apple ID that you used when downloading the i Tunes movies.
For the error, please check if your file name contains certain special letter which may also caused the problem.
For Windows version, you can see the option on the right of the panel under the preview window.
For Mac version, after loading file, you'll see the option on the left of the main window.
Because we do not update standard version for Windows any more and Plus version works much better and faster than standard version.
Please right click the program icon, and choose "Run as administrator" in the drop down list to launch the program.
Also, the i OS-based devices that support Air Play will cause this problem too.
If your Mac is connected with device like Apple TV, i Pad, i Phone or i Pod, please disconnect device and then try again.
Please open i Tunes Preferences by clicking "for Mac users.