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There are situations where signing in to the locally installed Teams App does not work at all or does not work as usual.
Take the following steps to wipe the local possibly corrupt Teams data and synchronize a new copy from the server.
Login to Teams online https://teams.microsoft.com/ succedes and you can work within the web client. (If not, there is a problem with your account and the following steps wont solve your issue.)
If your Teams App cannot be found, download it again and reinstall it.