I’m getting the same error as the OP, on 2 different development sites, both before and after the 4.0 update to WordPress. (Can’t point you to those dev sites since they are password protected until development is complete, but I’ve tried removing the passwords briefly to test this, and there is no difference – I still get the error.)
Very keen to hear of a resolution on this! ??
EDIT: Correction, it’s a 500 error only without the passwords. 401 otherwise, which I think is to be expected.