QuickTime Player and iBooks Author quit unexpectedly while using the MediaToolbox plug-in
I have recently installed macOS Sierra on a late 2012 iMac 27". I opened a file within iBooks Author (v2.5) that I was having no problems with when running El Capitan. iBA now crashes anytime I try to save the file, which contains embedded mp4 videos. The crash prompt says "iBooks Author quit unexpectedly while using the MediaToolbox plug-in.
I get the same crash prompt when I try to open any mp4 file with QuickTime Player from the Finder. I can open these mp4 files in other apps without issue (e.g., with Handbrake or Camtasia).
Any suggestions to clear this problem up are greatly appreciated.
iMac, macOS Sierra (10.12)