I have searched for any resolutions regarding scheduled refresh errors but haven't read any. I am getting the following:
Credentials are required to connect to the SQL source.
The exception was raised by the IDbCommand interface.  -1055784828: The database operation was cancelled because of an earlier failure.  -1055784932: The command has been canceled.. The exception was raised by the IDbCommand interface.
I have refreshed and reentered the the credentials multiple times to no avail. This is only happening with one of my reports. All other reports are refreshing as expected.
I solved a similar issue recently with a very similar error message.
In that case, one of the data sources in the report tried to connect at the server level, then the second step connected to the specific database/table. The account we use on our server only had permissions against the specific database therefore the refresh was failing. Everything worked when opening the report in PBI Desktop, as analysts have read permissions at the server level, so the issue was not obvious to find.
Not sure if this helps in your case, good luck in fixing the issue!
We've had occasional issues when we've modified the datasource configuration of existing reports.
In most cases, this was resolved by deleting and then re-deploying the report. On a few occasions even this failed with the only work-around being to re-upload the report with a slightly different name
Both setups work exactly the same in power bi desktop i.e. no exceptions. However, the exception of IDbCommand interface will be thrown if there were more than one query and at least one of these queries used different setup to the rest of queries.
Open Advanced Editor to ensure all queries are setup using the same format.
How to open Advanced Editor:
Edit Queries --> select the query you want to modify --> Advanced Editor (small button next to Refresh Preview)