Please update sample configurations files included with ebuild to include the option RequireValidShell off The default is to require a vaild shell for the user proftpd runs under. The .sample configuration file provided runs proftpd under the user proftpd. The ebuild creates the user proftpd with the shell /bin/false. The .dist sample configuration file runs proftpd under nobody which also has this same problem Proftpd fails with the misleading error login failed in this configuration.
Fixed, thanks for finding this!