There are several bug reports, this last beta has a java error in the Audio plugin. You may also notice it won't open in some browsers when the problem pops up. To test, delete the audio plug in and try exporting.
Revert back to 212.2 until the problem is fixed. to revert a saved project, open the caproj file in a text editor and change the version number to:
<saved-with-version>21202</saved-with-version>