Bumping this issue up. Have there been any updates on this issue? I am experiencing it as well, certain widgets in my dashboard randomly start to display "Data source error" and say "Cannot access data". This is not true, I can open these layers in a webmap, I can pull them into Pro, and I can call on them in scripts and they all behave normal. This appears to be something with the browser, however clearing my cache does nothing to resolve it. If I sign into the portal the dashboard was published in using Edge various elements display the warning. If I open the same dashboard in Chrome using my other portal account (each portal gets its own browser), the dashboard works fine and all elements function properly. There is no data source error. The data is all there and functioning. If I sign into my remote desktop and use the internet browsers on that machine, shocking that all the data is there and the widgets function. I would love to know how to get AGOL to refresh itself and find my data. It is a massive pane to have to sign into my RD to use the Edge browser there to make any edits.
This is different than the "Not fully configured" error that some users get. That issue is easy to fix by clearing your browsers cache.
