Outlook 2016 - Cannot copy the items. The operation failed. An object cannot be found

I have a user that likes to drag-and-drop attachments onto his Inbox of Outlook 2016.  This will create a new message with the attachment attached.  Recently, he started getting the error message: Cannot copy the items. The operation failed. An object cannot be found.  I have tried everything from deleting the .ost file, deleting search folders, creating a new profile, and pressing the Esc key.  Nothing has worked.  This issue is happening with Word, Excel and .PDF files, just to name a few.
K5-TechIT ConsultantAsked:
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Rutger BreedijkConnect With a Mentor Commented:
It is definitely related to an Office update.
We had some users with older versions of Outlook 2016, drag & drop worked without problems.
After using the update function in Outlook the client was updated and drag & drop stopped working.
Randy DownsConnect With a Mentor OWNERCommented:
Try this.

... something to do with Cached Exchange Mode.

Here are the steps I followed, and I was able to fix the issue on three of our PCs:

Go to Control Panel > Mail > Data Files
If more than one data file appears, select the main .ost file and go to Settings > Advanced
Uncheck the "Use Cached Exchange Mode" box and restart Outlook
At this point, drag-and-drop should work.  I went back through the same steps and checked the "Use Cached Exchange Mode" box and restarted Outlook..  
Rutger BreedijkConnect With a Mentor Commented:
I am having the same problem with two different accounts/computers.
Maybe coincidence but looked like it stopped working the same day I had an Office update (januari 5th)
Office365 ProPlus 16.0.8730.2165

Tried switching between cached and uncached but did not solve the problem.
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K5-TechIT ConsultantAuthor Commented:
I have tried Randy's suggestion with no luck.  It also seems that the Add to Dictionary function is now greyed out as well when you try to add a word to the dictionary from spell check.
Randy DownsConnect With a Mentor OWNERCommented:
The message seems to indicate an issue with permissions. Maybe this "fix" will help. Apparently this sort of bug has been around for years.

Close out anything Microsoft Office related. Even close out any that remain open in the notification section when you close them, like OneDrive.
Open the Start Menu
Type "Credential" and select "Credential Manager" from the results.
Go to the "Windows Credentials" section.
Remove all the credentials that have POP, SMTP, SharePoint, Office 365, or just Office in their name. Honestly you should be able to just remove all the credentials listed here with no ill effects. Worst case, you may have a few pop ups later asking you to re-enter credentials.
Log out and back in just to be on the safe side.
Reopen Outlook, typing in credentials if asked.
Try attaching an attachment now and see if the issue persists. I have noticed that Office 2016 seems really bad about hanging on to old or corrupt credentials, or just messing up credentials in general.
K5-TechIT ConsultantAuthor Commented:
I think Randy is on the right track to fix this issue because the issue started after the user changed their Microsoft password.  I tried doing what he suggested but I still get the error when trying to drag-and-drop attachments.  Is there a way to maybe do a complete uninstall of Office that would remove all credentials?
Randy DownsOWNERCommented:
You could uninstall Office. There is a link for
. Perhaps that's a better removal than simply uninstalling via Control Panel. I would also check that there are no files left in Programs & clear caches & such with cCleaner.

Then Clear Credential Manager & create a new User Profile after rebooting.

Perhaps you can set Office to not install updates for now if it works initially.
Rutger BreedijkConnect With a Mentor Commented:
Not sure what happened but it started working again. According to update list these updates were installed last few days :

Update for Windows 10 Version 1703 for x64-based Systems (KB4023057)
Security Update for Microsoft Visual C++ 2005 Service Pack 1 Redistributable Package (KB2538242)

Office version is now :
Office365 ProPlus 16.0.8730.2175
Randy DownsOWNERCommented:
Split the points even,y with  Rutger Breedijk. Apparently an update caused this issue & fixed itself. Not sure if author's problem was resolved the same.
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