Unfortunately, this is a bug with Cinema 4D R21 and happens when attempting to use Cinema 4D R21 on another machine when the license is already assigned.
Example: You sign into the Cinema 4D R21 License Manager on your office computer. You leave work and come home to work on your home computer. When you attempt to launch Cinema 4D R21 on your home computer, you get a Script Error because Cinema 4D R21 is assigned to your office computer.
The workaround is to log in to your MyMaxon account on my.maxon.net and release your R21 license. You can release the license by clicking the small "x" in the upper right-hand corner of the license (screenshot below). Once released, you should then be able to use your R21 license on a new machine.
If you continue to run into errors after releasing the license please open support ticket here: