I have the exact same issue right now. Until a couple of days ago I was using the dashboard without issue, now for the last couple of days and for several gadgets (not all) I get the same error message that Vasiliy had. I tried deleting them and reinstaling them, but that didn’t help.
This hampers my ability to create the reporting that my company expects me to produce.
In the end I was able to determine that the issue was caused by the browser (Chome). If I used a different browser I would not get the error message. I removed Chrome and reinstalled it and then the issue was solved.
No idea what exactly went wrong, but I know what the issue is now and how I can fix it if it happens again.