Hi @Joris Slingerland ,
I can understand that you've already tried methods like using SARA, deleting appdata and reinstalling Office, but Outlook profile is not changed in these steps.
So, in case it's an issue with the current Outlook profile, I'd recommend trying to create a new Outlook profile, adding these accounts one after another and see if there would be any difference in the new profile. For detailed instructions on how to create and switch to new Outlook profile, hopefully you can find the link below helpful:
Overview of Outlook e-mail profiles
If the error remains, I'd suggest creating another new Outlook profile and adding the 365 mailboxes first, then adding the outlook.com account along with them. This helps narrow down if it's the issue when outlook.com account is added as the primary account of the profile.
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