Thanks Kay for your quick reply!
I discovered this morning that the issue was of my own making ... I'm testing a new way to retrieve Fan Chart data to get data about living WikiTree members when logged in (according to privacy settings) ... and my testing code was generating corrupted cookies on my local laptop, which generated this error. It'd be so much easier if the error message was more explicit, like "You've baked some Bad Cookies, Dumbie !" .... oh well.
Glad that the actual Fan Chart is still working - as is the rest of the apps. Phew !