Not sure what would cause that error message, nor have I experienced it. Looks like other people have seen it: https://www.google.com/?gws_rd=ssl#q=gss_init_sec_context+failed+with+%5BUns...
I found no way to get rid of this, although everything seems to work fine.
Red Hat need to push out an update to samba4 and fix bug 10604. It's highly irritating, and the workaround https://bugzilla.samba.org/show_bug.cgi?id=10604#c30 doesn't. The patch from https://bugzilla.samba.org/show_bug.cgi?id=10604#c17 does.