Hi @Luc V ,
Welcome to our forum!
What's the popup? If possible, please provide a screenshot of it for our better research. (In order to avoid the privacy leakage, please remember to hide your personal information.)
And is everything OK in Gmail on web?
If everything is ok on Gmail.com, considering that it may be caused by the damage of the profile/data file in the Outlook client, it is suggested that you could try to create a new profile (Control Panel>Mail>Show Profiles>add) and add the account for testing to check if there are some differences.
If the answer is helpful, please click "Accept Answer" and kindly upvote it. If you have extra questions about this answer, please click "Comment".
Note: Please follow the steps in our documentation to enable e-mail notifications if you want to receive the related email notification for this thread.