Hi, Gundula. I am an independent advisor, trying to help other users in the Community with my experience in Microsoft products. Please note that I am a user like you and don't work directly for Microsoft.
I’m sorry you are having trouble with the scroll bar in Microsoft Teams. I deeply understand the issue has affected your work, rest assured I will try to provide you with a solution.
I understand you already switched back to Old teams, while that is also encouraged, I would be providing you with a solution should you decide to have a change of mind.
In the article on https://answers.microsoft.com/en-us/msteams/forum/all/scroll-bar-in-microsoft-teams-has-disappeared/db197c69-8acd-41b1-8750-e181cfc3d49d Nik gives the following steps below as a solution:
"Thanks for your post on the community forum. I'm Nik, an independent advisor here to assist you with this.
To narrow down the issue, please check if you see a similar experience on the MS Teams web app.
Go to access 'Teams Web App' https://teams.microsoft.com, use the "Use the web app instead" option to open teams in a browser(preferably Chrome or Edge) and check the outcome. It will help to isolate the issue with the desktop app.
If the issue is specific to the MS Teams desktop app, then try clearing the app cache by these steps and try again: https://www.uvm.edu/it/kb/article/clearing-teams-cache/ "
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I hope this helps you. Let me know if this contradicts what you need. I remain helpful in answering any more of your questions.
Warm Regards
Femi