Dear RT 5,
Greetings! Thank you for posting in Microsoft Community.
I'm sorry you get error when using PowerPoint desktop application but we can work together to narrow down and resolve the issue.
For the situation you encountered, we deeply understand the inconvenience caused and apologize for it.
May I know if you have tried removing Office credentials from Credentials Manager and Registry Editor? If not, I will appreciate you can try the troubleshooting steps below and tell me the result.
1.Sign out from PowerPoint app and close all Office apps.
2.Open Windows Credential Manager > Windows Credentials > Generic Credentials > remove credentials related to Office.
3.Clear Office credentials from Registry Editor:
Open Registry Editor, go to HKEY_CURRENT_USER\Software\Microsoft\Office\16.0\Common\Identity\Identities, delete the whole Identities folder. (Please back up it before you delete it.)
4.Reopen PowerPoint, sign in your account, open a SharePoint file and see if issue happens or not.
If issue still happens, considering that you mentioned this issue only occurs in PowerPoint app, you may try removing cached files from PowerPoint via steps below:
Close all files. Open PowerPoint app, go to File>Options>Save>scroll down to Cache Settings, click Delete cached files and click OK. Reopen a file and see if the error appears or not.
And I will appreciate if you could share following information, as it will help narrow down the situation and do further troubleshooting:
1.Does the error appear when you edit in a specific PowerPoint file or different files?
2.What is the version of your PowerPoint app? I'd like to test in same version as yours.
Please go to File>Account, then capture a full screenshot of Product Information.
Thank you for your time and cooperation. Hope you have a nice day!
Sincerely,
Tina | Microsoft Community Moderator
