There are the steps to represent the bug.
1. Open event list in category view (eventsbycategory). I have something of about 14 events so it is 2 pages.
2. Choose a specific category from the drop-down list. In my case there there are only three events in the chosen category so there is only one page.
3. Choose «all categories» again from the drop-down list. Now back to 2 pages.
4. Click on the 2nd page link.
Now instead of second page of events list we are in «by month» mode and see the calendar.