I had the same problem and searched for hours trying to find a solution, rebooting, etc.
Here is what worked for me.
1. When it asks to reconnect media, make a note of the file names it is looking for.
2. Copy and paste those files into the folder you think it wants it to be in and/or the folder the project is located in (I did both, although I really just took a shot in the dark as to the original folder)
3. Restart Motion by opening the project in question.
Not sure if having the file in the project's folder was the answer or guessing the original folder, but I would bet the media needs to be in the same folder as the project.