Before adding the missing route=hiking tag to the relation already had lots of flags, but after the tally got too long for me taste, the oddest, thinking or wanting to have a route=ferry on the route, like 1000-2000m SLM.
Yes, the hikers like mapping ways through and over buildings and attach them to sides, but that’s of later concern.
Virtual six-pack for the one who knows.
PS, not first time seeing this route=ferry warning and could not find answer.
It would be much easier to answer had you linked the objects in question, instead of letting us search for ourselves…
So, it has something to do with Relation: (SI-C P03-B) Rivisondoli - Scanno (12605535) | OpenStreetMap .
While I’m not familiar with JOSM, I happen to work on a different database rules validator in real life, and so I know that validation messages written by programmers for programmers can sound really weird. (see below*)
Based on that background, I guess that JOSM does not like that the relation in question (route=mtb) is part of Relation: traccia di collegamento (12601250) | OpenStreetMap (route=hiking). The error message mentioning highways and ferries is probably a smokescreen, since a route=ferry is an exception which may be part of other kinds of routes.
*) The error messages generated by our software are, approximately, “Invalid value ‘location=work’ for condition ‘suit=swimsuit’. The valid values are {beach}”. In human language, that means “You may not come to work in a swim suit”. 
Oops, sorry, it’s actually Scanno-Rivisondoli-Scanno or v.v., works both ways, a roundtrip ** but mapped as 2 separate routes. Ignored it in past as a bug in the bug validator but now with all those lights on wanted to know more. And more than many do it on an E-mtb.
** Side question, would the roundtrip=yes apply to the route relations even when mapped separately? Still on the same bike same as these split bus routes for out and in are often split.
