It is a feature of
helm-find-files, when your point is on URL, such as "http://example.com" and "firstname.lastname@example.org",
helm-find-files guesses you want to "open" these link. To avoid this, you can move your point from "C@d.yaml" to "~/A/b/", then it will thinks it's a file path at point.
If you don't need or want this feature at all, you can turn off via
(setq helm-ff-guess-ffap-urls nil)
helm-ff-guess-ffap-urls is a variable defined in ‘helm-files.el’.
Its value is t
Use ffap to guess local urls at point in
‘helm-find-files’. This doesn’t disable guessing filenames at point,
see ‘helm-ff-guess-ffap-filenames’ for this.
You can customize this variable.