Whenever I try to add a hosted feature layer to an existing Insights workbook on ArcGIS Online, I get an error that something went wrong. However, if I add the same hosted feature layer to a new workbook, it works just fine. I've also tried uploading a csv file to an existing workbook and get the same error, but if I upload the same csv file to a new workbook it works. Any reason why this would happen or things I should check?
I am not sure why you are experiencing this, I can't seem to replicate this problem yet but we will keep trying. For now, maybe you could you try duplicating the workbook and then seeing if you can add data to it? You could also try exporting the workbook and then importing it?
May I ask, are you seeing this problem with just one workbook or is it any existing workbooks you have?