Data refresh through PBI Gateway keep prompting credential missing incorrectly

I am having exactly the same problem, with the same error code -2147467259 and error message. I also have a full list of green ticks. The report had been successfully going through the scheduled refresh until I made some minor changes to it on Saturday morning. I have run some simple test versions, and the stage that seems to create the problem is any merge involving Excel or csv files, despite them being within the common folder used for the on-premises gateway. Without such merges, all of the data will refresh. I have another report with some of the same data and merges that has not been amended since 6 August, and that is continuing to refresh successfully. Is is something to do with the August update of Power BI?