Tracking, updating, and organizing tasks within Microsoft Teams or integrated apps
Hi Rene,
We appreciate that you posted back and shared the detailed information with us. We also appreciate that you spent your precious time doing those tests.
Per your description, it seems that you have tried removing this account and re-adding again, however, the same problem still persists. And you have checked the existing plans, you found this problem occurs on all planners and even new plan.
Given this situation, this issue may be related to specific user account. We do understand the inconvenience caused apologize for it. This issue may be related to your account and may need to be checked from background. The best way to troubleshoot the issue is to report the issue to the related Team. So we suggest you contact the administrators in your organizations and open a support ticket in the Office 365 admin center via the steps in the article for further help. The support team there has the correct escalation channel for this case. This is also the most efficient way in handling this kind of issue.
Your understanding and patience will be highly appreciated.
Best Regards,
Sukie