The message actually says:
“Several errors occurred during data refresh. Please try again later or contact your administrator.”
SessionID: 1b80301e-3898-417a-af9c-2e77ec490728
[0] -1055784932: Credentials are required to connect to the SQL source. (Source at SQLServerName;DBA_Pro.). The exception was raised by the IDbCommand interface.
[1] -1055784932: The command has been canceled.. The exception was raised by the IDbCommand interface.
[2] -1055784932: The command has been canceled.. The exception was raised by the IDbCommand interface.
In my case the cause of the problem was very silly thing. PowerBI Server assigned only one data source connection string to my report, while in my report I had two data sources with only the difference in a Database Name capitalization:
This was a weird scenario.