Hi @Ivan Bútora
Great to know that the issue has already been resolved and thanks for sharing the solution so that others experiencing the same thing can easily reference this! Since the Microsoft Q&A community has a policy that the question author cannot accept their own answer. They can only accept answers by others, I'll repost your solution in case you'd like to Accept the answer.
Case Title: Wrong Icon of the Teams app/list in sidebar
Issue Symptom: the icon of the "Teams" app/list in the left sidebar is not showing properly - instead of the original icon, I just get a strange square.
Resolution: As Teams is currently migrating to New Teams, Teams Classic may experience some temporary issues. This issue will be automatically fixed after waiting for a while.
Thanks for your understanding and support.
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