This will be because your flags
setting in the global config has the debug bit (2) set. If GC_DEBUG is set in the global configuration this feature will be enabled.
These are the flags:
-define(GC_TTY_TRACE, 1).
-define(GC_DEBUG, 2).
-define(GC_AUTH_LOG, 4).
-define(GC_COPY_ERRLOG, 8).
-define(GC_BACKWARDS_COMPAT_PARSE, 16).
-define(GC_LOG_RESOLVE_HOSTNAME, 32).
-define(GC_FAIL_ON_BIND_ERR, 64).
-define(GC_PICK_FIRST_VIRTHOST_ON_NOMATCH, 128).
-define(GC_USE_FDSRV, 256).
-define(GC_USE_OLD_SSL, 512).
I think that in the yaws.config file this is actually un-set by setting the debug = false flag in the global part.
Updated - the debug parameter is part of the env settings of the app in the yaws.app file, so adding something like
-yaws debug false
to the real command line should work