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.
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.