I originally posted the problem to the Bubbletimer module issue queue, Unable to save (pressing Save button has no effect).
The error can be seen be seen on the server by looking at the Apache error log:
sudo tail -f /var/log/apache2/error.log
Try to submit the 20 activity form, and nothing happens on the Drupal /bubbletimer page, but this error shows up in the log:
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