We’re very sorry to hear that your Core might have a corrupted database . There is a solution though; the steps are listed here, just as @xxx has shared before (thanks @xxx ):
Well after a fair amount of hair-tearing and stuff. I eventually stopped going round in the log-in loop which kept saying I was already logged in to the Nucleus +. Instead, as a desperate measure, I clicked the option to deauthorize the Nucleus+, and it then just logged me in without any further ado. Clearly deauthorize does not mean what I think. Very unclear what the problem was or whether it has been fixed.