Hi Matějka ,
Thanks for your post on the community forum. I'm Nik, an independent advisor here to assist you with this.
You might be experiencing some issue due to recent MS teams service degradation; please find more details below. If you have admin access to your tenant, then you can find more information here: https://portal.office.com/adminportal/home#/ser...
I'll update you as soon as Teams services back online. Currently, the Microsoft product team is working on this.
Latest message, April 27, 2021, 6:47 PM
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Title: Can't access Teams
User Impact: Users may be unable to access Microsoft Teams.
More info: Users able to access Microsoft Teams may experience degraded performance with multiple features.
Current status: We're analyzing system telemetry to isolate the source for the issue. In parallel, we're reviewing recent changes made to the service as a potential root cause.
Scope of impact: Initial indications suggest that any user could be affected by this issue.
Next update by: Tuesday, April 27, 2021, 8:30 PM (12:30 PM UTC)
Regards
Nik