A group of Microsoft Products and technologies used for sharing and managing content, knowledge, and applications.
Dear respected Henk,
Good day! Welcome to Microsoft Community.
We went through your post carefully and it seems like you have done the Clear Account Cache many times and also the reinstallation.
If so, may I know if the error appears when trying to sign into the SharePoint mobile app on your iPhone and when you enter the account password and MFA, and it shows the error message as shared in screenshot and cannot access the application.
If my understanding is correct, we did receive many other users reporting the same issue, normally on iOS environment. And some users resolved issue by remove account credentials from browsers. You may also try to download Edge.
Appreciate your understanding that sometimes the initial reply may not resolve the situation very soon. However, we can work together to narrow down and resolve the situation. Please kindly post back and provide the more information, then we can focus on your specific scenario and provide specific suggestions.
We'll be here for you to wait for your updates. I appreciate your time and effort on checking these above and thank you for your patience and cooperation. I hope that you are keeping safe and well!
Sincerely,
Mia | Microsoft Community Moderator