I had created a lot of scheduled reminders on my iPhone. When I click on “Show Completed Tasks” in app, it shows over 18,000 completed reminders and some of them are blank in iOS 11. Then I select a blank one on the top row to remove it, the app keeps crashing. I have to force close the reminders or restart the device. I log into icloud website in Safari browser and use the “Clear Completed” option to delete all completed items, but they appears again in the next time I sign in.
I go to disable the iCloud feature on both of my iPhone and iPad. After I perform the same mentioned steps and manually delete each completed reminder in iOS 11, nothing takes effects with icloud.com. It seems that the Reminders can not handle a large number of completed items in iCloud, and finally I find a way to get rid of them completely.
* On your Mac, once you’ve logged into icloud.com using your account, go to reminders and create a new list.
* Select all the uncompleted items by clicking on your first uncompleted reminder and then press shift key to click on the last uncompleted reminder. Then drag them over to the new list.
* Click on “Select All” from the Edit menu to select all completed items, and delete the old list. Now the problem is resolved completely.
However, you should also face the little issue. When you complete an item, Reminders responds promptly and notify you. You need to tap on the Complete button to close it. For the fully blank Completed Reminder, I think it is a bug in iOS 11 and hope the next update fixes it.