Hello Iñaki Anso,
Thank you for reaching out.
Can you please share with me a screenshot of his error message? You can paste directly in the reply body or use the picture icon .
->Also, can you log into the new teams on the web with the same account with no issues? https://teams.microsoft.com/v2/
->Can you log into other office applications without issues?
In the meantime, please try to run teams as an admin and check if you can successfully sign in. Search for the teams app >Right click >Run as admin.
->Also, if you are connected through a proxy, VPN or antivirus, please turn these off or bypass them and try to log in again.
Looking forward to your response.
Best Regards,
Ashraf.