I have the same bug with ver 6.4, enable is gray with winbox but ok via the http web site, I have in stock 4 mikrotik with ver 6.4 they have the same problem, I checked with my RB951G-2HnD too, I reboot, do reset factory always no enable check available with winbox !
nothing I can do with winbox, I tried my 4 devices with ver 6.4 same problem, if go to IP-> smb the enable is gray, by Http or terminal working fine …
I can do the setup by 2 last ways, but it will be good if mikrotik can confirm if that is a problem ???
The weird thing is with winbox I can’t change anything after the setup, but the share and user I can do everything ! (not → enable, domain, comment, allow, interfaces)