This name is a synonym of Knautia dipsacifolia subsp. lancifolia (Heuff.) Ehrend..
The record derives from WCSP (in review)
(data supplied on 2012-03-23)
which reports it as a synonym with original publication details: Verh. K. K. Zool.-Bot. Ges. Wien 8: 127 1858.
This name record (from WCSP (in review)) was not included in version 1 of The Plant List. It is possible the same name was included from a different data source, to check this you can search for Knautia sylvatica in version 1 of The Plant List.