Export - cannot select track in checkbox

I’ve been using Ardour for most of this year and it’s a complete godsend. But, I just upgraded to 2.1 (rev 2496) and now I cannot select tracks in the Session-Export dialog. The check boxes are unresponsive, even after changing several of the options. Now, I can work around this issue if it’s a bug to be fixed sometime, but wanted to make sure I wasn’t missing something.

Thanks so much!