Write in Word, suddenly Outlook window is active - irregular but extremely annoying and dangerous

Hi,
I have the following problem. One of our customers is a law firm and they use MS 365 Office applications on normal Windows 10 clients and of course special law software.

We have known for a few months now that the following happens very irregularly on some of the computers:

A user writes a text in Word, suddenly Outlook is the active window and emails are accidentally deleted unintentionally and sometimes unnoticed.

Microsoft Support was already there, but the problem simply cannot be reproduced, but it is now and understandably causing great uneasiness among those affected and the executive floor.

I then spoke to one of the special software providers and asked him if he had ever heard of something like this, and he actually had a single case where another IT specialist described exactly the same behavior. The problem went away for him afterwards, but he probably didn't have a "real" solution at the time either, but rather luck.

Has anyone here from the forum ever had the same problem and a real solution or solution?

New profiles and repair installations of Office do not help. The customer's suggestion of completely deleting and reinstalling Office was not well received by the customer, since I then have to set up Office again on the computers of several users.

Greetings

Sebastian