Had this problem on a PC today – Outlook was having trouble opening .MHT file attachments from a contact item. Any attempt to open any .MHT file would result in an error message stating “Cannot save the attachment xxxxxxxxx.mht. Can’t create the file. It then asked to open the folder / file in question and check security permissions. This could happen for any attachment type, but in my case it was .MHTs.
Anyway the quick solution I found is to browse to the “OutlookSecureTempFolder” and remove all files listed in that directory. Close and restart Outlook and everything should be working again. Credit to this page where I found the solution.
1. Open REGEDIT.EXE and go to Edit -> Find… In the Find dialog box type “OutlookSecureTempFolder” without the quotes and locate that registry key.
2. That key will contain the actual folder location, and will look like:
C:\Documents and Settings\%USER_NAME%\Local Settings\Temporary Internet Files\OLK#\ (where # is a random letter or number)
3. Copy the location of that folder.
4. Click on Start -> Run… and paste the folder location from step #4 then click OK.
5. Windows Explorer will open that folder. Please, delete all files present.
6. Restart Microsoft Outlook and you should be able to open your attachments.
A quick update to this – the error occurs when you have the same document saved in this folder 99 times. This seems to be the maximum limit of times you can have an attachement from Outlook saved into this “temp” folder. Each time you open a file with the same file name from Outlook a new copy is saved here. For example (filename.zip (1) filename.zip (2) etc.)