try using #'swank:swank-debugger-hook instead.
Is it ok to pass nil as the "hook" argument? So far, the hook seems not to be used, but this does not tell me what I should put there to be safe :)
(setf araneida:*restart-on-handler-errors* #'(lambda (condition) (swank:swank-debugger-hook condition nil)))
Frédéric