Error after joining teams meeting

Hello,

At our company we recently switched to O365 and respectively Teams, currently I'm working on an issue with the Outlook teams add-in. The problem is: once the person receives a mail with a link to join a Teams meeting, he gets prompted to pick the Teams app, but then receives the following error.

The Windows version is a Bulgarian one, but I believe you know what it says by the way it looks:)

please for your assistance!