Error when creating a new data source on Report Server

Hello!

We are trying to create a new data source on Report Server, but the following error occurs when we enter the username/password and click "Create" button: 

User name and password are correct, because when I'm trying to fill in the same username and password in other data source just to check if the connection is successful, everything works fine with this username and password:

Could please anybody tell where the problem lies or where to start looking?

Thanks!