Hi, Ann-Kathrin Fischer
Good day!
Thank you for posting to Microsoft Community. We are glad to assist you!
According to your description, it sounds like you're experiencing some frustrating issues with the Microsoft To do web application.
As specific support channel resource for certain different support scope and attributes. Since we're focusing on Outlook and Teams, we're afraid that we could not give you an appropriate suggestion here. For this given issue is relates to the Microsoft To Do and we do not have enough testing environment and fixing tool for this so this is really out of the scope of support in this community.
Therefore,would you please place your query in the right place in here -Category: Microsoft To Do | Microsoft Community.
For this Microsoft Microsoft To Do tech support experts in here who are responsible for consult about this given issue and will provide you further help.
Note: For Microsoft To Do's community official language is English, please be kindly to translate your query in English and then seeking for the expert team for further assistance.
Thank you for your precious time and your understanding would be highly appreciated.
Sincerely,
Alex | Microsoft Community Moderator.