Nothing mentioned here worked for my wife's iphone4s.
She even did the reset all settings thing, which created a bit of calamity because then Siri and Siri location services were lost. After bring back Siri, I went to work.
What I noticed was when I went to Settings -> iCloud and turned off Reminders, the Reminders app worked again.
When I turned Reminders on again in iCloud, it stopped.
So, for kicks I logged into her iCloud.com account. Her settings were all there. All 167 of them!!!!!
Therein lies the problem. She was marking reminders as completed, but not deleting them. I don't think iCloud.com, even in an era were terabytes cost pennies, can handle 167 reminders.
So, this was what we did:
- Went back to the iPhone, Settings->iCloud and turned off Reminders
- Went to the reminders app, and entered the two (!!!!) reminders she had not set completed
- Went back to iCloud.com and created a second reminders list (this is needed for the next step, because iCloud.com requires a minimum of one)
- Deleted the Reminders list from iCloud.com
- Went back to the iPhone, Settings->iCloud and turned Reminders back on
- Went to the Reminders app, and saw a pleasing message that said her Reminders would be merged with iCloud
- Went back to iCloud.com, refreshed the Reminders page, and in a few seconds, the two reminders she entered on the phone showed up.
- Deleted the second list reminders list from iCloud.com
- Went back to the iPhone and verified that the reminders app still worked.
- Taught her how to delete reminders when done.
Hope this helps someone else.