Hi Sebastian Breton
Thank you for posting in Microsoft Community
Please confirm what version of Outlook you're using?
Does this occur when using both Outlook web app or Outlook desktop client?
Also confirm if this only happens when you try to open messages from your inbox or other folders too?
If you're using Outlook web app kindly log into your Outlook using the InPrivate window which can be done by navigating to the top right of your browser on the more options (…) and try to open your inbox messages.
You may also try to use another browser and see if the issue persists then.
Regards,
Momo