Backup 'Continue' button vanished

Here’s an odd one:

I’m currently migrating courses from a 1.9 Moodle site to a 2.3.  I’ve almost finished but suddenly, I can no longer make backups of courses.  This screen shot shows a course midway through the backup procedure. There should be a Continue button at the bottom of the screen, but as you can see, it's gone!  I've tried with different browsers and the same thing happens.  Refreshing the screen doesn't work either. As I'm part way through migrating courses, I'm perplexed as to why this should suddenly happen.  Anyone else had any similar issues, and if so, what did you do to fix it?

Thanks folks!