Some time ago, my Outlook 2007 kept prompting me with the following error: “Cannot turn off the reminder. You may be reminded again. Could not open the item. Try again.” After experimenting for quite some time, I finally found a solution somewhere on google. Unfortunately, I only noted down the solution, but not where I found it – please let me know if you think I stole it from you. :-)
edit 2017-06-04 15:43: image removed, source dead
Here’s how we fix this problem:
Download MFCMAPI tool from codeplex.com/MFCMAPI, then:
- Run MFCMAPI on the desktop
- Go to “Session” ->Logon and Display Store Table
- Select your profile
- Doubleclick the affected Mailbox
- Expand “Root Container”
- Right click “Reminders” and select “Open Contents Table”
- New Window Launches with Title “Reminders”
- Select the reminder items listed there, Right click and select “Delete Message”. This will delete not only the selected reminders, but also the items they belong to, so you might want to re-add them afterwards.
- Close MFCMAPI
… and you’re done. Leave me a message if this post was any help to you, or – more importantly – if it wasn’t. :-)
 Thanks to Stephen Griffin for the link to the up-to-date version and the heads-up.