We are newly seeing 404 errors from the /rest/api/2/filter/search endpoint, which is very odd because that endpoint should never return 404s. Anybody else seeing the same thing?
I faced the same issue. Did you resolve it ?
I don’t remember now. I have a feeling, though, that the problem was with the access token that we were using. If you don’t authenticate properly, then the search is evaluated in the anonymous context, in which case the filter you are searching for cannot be found. I suppose that could account for a 404.