@David "I tried unrooting, but this cannot be done as it cannot access /system where KingoRoot apk is stored."
-Did you try to Unroot going on their "Superuser" app?
Superuser -> settings -> Remove Root.
That should do the job of uninstalling Superuser/Unrooting. It would still leave Kingo, but you can uninstall it from normal android settings -> Apps panel.
If you lost the root but their Superuser app is still installed, i would open kingo and re-root, then "Remove Root" after :-p
If the above fails and i could not remove it, i believe a factory hard-reset from the stock recovery options should fix it - have you done it ? you said "reboot" i dont know if thats what you meant.
(back up your data first, make sure you remember your credentials).
I would do one, and If it still did not solve the problem i would flash the SAME stock firmware i was using, then move my data back/reinstall apps.
And yes, like people say on the comments here - and also from personal experience - these "one click root" apps have the advantage of being really easy to use (maybe not the ones in chinese language lol) and work GOOD for lots of different brands/models... the disadvantage is these "half-rooted" cases (devices with Android 5+ ?) when the SU binary is not fully functional and will not work right for many apps that need SU permitions.
I had many similar mounting issues and some others with yet another chinese 1click app called "360 super root". UNfortunately, it was the only one that worked for my device so far. Kingo, king, iroot, wmroot all failed. This "360root" (aka permroot) was the only one to work, but its SU was buggy like yours seem. Fortunately now i managed to install a good root via custom recovery... root now works 100% for the stuff i need.
There IS a way of replacing Kingo's Supersu by a different/better SU app without losing the root Kingo did. There is a paid app that does this trick that is called "Super-SU-me" and also a manual/free way to do it, but dont ask me how :-/
By the way, what is your device brand/model and android version ?