Outlook 365 keeps corrupting PDF and Office files

Since April we started a new EA agreement with Microsoft and added all of our users to E5 licenses of M365 on top of the E3 licenses they already had - we were told that the E3 licenses would disappear anyway.

We don't know if that is related, however since mid-April we started to have numerous users who find their email attachments to get corrupt in Outlook. Whether it's with Outlook 2016 or 365, and Windows 10 or 11 Pro, the symptoms are always the same: the size of the attachments decrease significantly and they cannot be opened in their usual application (Adobe PDF Viewer, Word, Excel, PowerPoint, etc.).

Attachments in emails are oftentimes received already corrupt and are therefore unusable, sometimes they are good to be viewed or opened, then later on get corrupt. Sometimes

the attachments get corrupt when their related emails get forwarded to other users (who also find corrupt attachments in the emails they received).

We noticed that Outlook sometimes get even in a state where no file can get attached to a new email, or no attachment from a received email can get saved on any drive.

Whenever a user tries to open a corrupt attachment, an error message similar to this appear: "there was an error opening this document. The file is damaged and could not be repaired". No useful details get logged in the Event Viewer, all we see at the time of the error is "An unexpected error has occurred".

We tried the SaRa diagnosis program on some users' workstations with no luck, the SaRa program gets to vanish before the issue is seen again.

We tried to run Desktop Outlook in Safe Mode, the issue has not been seen. We tried to disable all of our add-ins one by one in normal mode, the issue was seen with every add-in being disabled....

We have hundreds of users and are out of ideas. Can you please help.