- Quit the Mac App Store.
- Navigate to /Applications/Utilities/
- Open Activity Monitor.app
- Search for "storeagent".
- Select it, choose "Quit Process".
- When asked if you really want to quit the process, choose "Quit".
- Open the MAS and try to download the app again.
Note: Before you do this, you can also try holding Option, and it will give you a chance to "Cancel" the download, but this didn't seem to fix the issue for me.