I am trying to configure an Event to repeat on every second Sunday but upon saving the event it shows incorrect dates in the Confirmation screen where it shows the same start date for each month and upon confirming it gives an error stating "Invalid day name". I tried by giving other day and it is working correctly but not for "Sunday". This was on CiviCRM version 5.10.3. I tried by upgrading to 5.13.2 but the issue was same in that also. I am not sure what I am missing. Kindly help me to fix the issue.
I'm thinking the invalid day name error is the result of the start date of June 10, 2019 which is a Monday, not a Sunday (note that in your screenshot, there is no date listed for the "Original" event). If you change the start date to June 9 (which is the second Sunday in June), then the repeat function works as expected (on 5.10.alpha1):
Hope this helps, Tamar