Using classic Outlook for Windows in business environments
Edit : the solution below was only a temporary one. :-(
I may have found a solution to this recurring and very annoying problem.
It starts from solution 3 above.
Here is the path I followed: File > Account Settings > Account Settings. I then selected my email account and clicked Change (or Modify) in the list of options above**.**
Then under the box stating "Use Cached Exchange Mode to download email to an Outlook data file ", I reduced the time span under which Outlook is requested to download the emails left in my inbox. It sounds that if you ask to download emails since the beginning, then you are in trouble. You need to find the optimal time span, between a few days and a few years, depending I suppose on the size of your inbox.
I tried to unchek the "Use Cached Exchange..." box, and at first it worked well, but after a few days it provoked some instability.
Hope this will work for others.
Ps : I am using Office 2021.