The documentation is not clear on this particular nuance, as it is not the slash that cannot be accepted, but specific formatting that must be used. For referencing multibranch jobs. In order for this to work, you need to place the "/job/" between your two levels of the jenkins job (e.g.):
Also, 'branch' part should be URL-encoded (e.g.):
'feature/no-manual-step' -> 'feature%252Fno-manual-step'