I recently allowed OneDrive to back up my documents to the Cloud.
After the uploading had finished, I got an error when I tried to download my emails.
The error was “Outlook .PST cannot be opened”
The .PST file is the email file that contains all of the information of your Outlook emails including the emails and attachments.
For some strange reason, MS chooses to create the “Outlook Files” folder in the Documents folder, instead of the C:/ root of the drive.

OneDrive does not back up the root of the drive it only backs up your Documents; your photos and your desktop.
When I looked where Outlook was looking for my .PST file, I saw that the path to my .PST file was now in OneDrive.
So as part of the synching process, OneDrive locks the .PST so the Outlook programme can’t access the .PST file.
The only way to resolve this is to move the PST file from the Cloud back onto your computer but not in the Documents folder.
I suggest that you move the PST file to the root of your drive.
After you do that and you open Outlook, you will get an error that Outlook can’t find your .PST.
Point Outlook to the .PST on your drive root.
Lastly, you need to close Outlook and access your Outlook email setting via the “User Accounts” link in Control Panel.
Click the Mail (Microsoft Outlook) link at the bottom of the list.
Once in your email settings, click the “Change Folder” on the bottom left corner and add a new folder such as test. Click this test folder and then click ok to close the email settings.
Now open Outlook and your problem will be solved.
Move any emails that have gone to this test folder and then close Outlook and again go back into the email settings and change the download email back to the inbox.
Finally open Outlook and it should be working correctly and you can now delete the test folder that you created earlier. When I opened Outlook I saw that the test folder has been removed for some reason.
Please note that you must manually back up your .PST file every so often as it will not be protected.

One thing that I have noticed, is that you will need to redo your email filtering as any rules that you have entered previously will not work after this method.