You may have found the solution by now, However, I had the same issue, Our devices were running MobiControl V13.6 and I found that after I upgraded the MobiControl Agent to 13.9 the options were no longer greyed out and I was able to upgrade the OS on the devices.
I did have to perform the upgrades manually using Remote Control, I was not able to perform them via script

Let's assume that your Android-Enterprise device is enrolled in device-owner mode. 

What are the version and build numbers of your device agent?

Have you deployed any feature-control profile payload onto your problematic device?

Hello Raymond,

the devices are not enrolled as android enterprise. They are enrolled as android+.
I also revoked all profiles from my device to garantuee that no profile is blocking the update. 

Agentversion 13.5 Build 1088

For Samsung devices running Android+ device agent,  OS upgrade option in Settings will not be grayed out without any profile from MobiControl.  

Have you power-on reset your device to see any difference after revoking your Android feature-control profile? 

Have you or your colleagues sent any special MobiControl script to the devices?

What are the version and build number of your server?  Have you used any Samsung Knox products (KC/KME etc) on your devices?

I deployed once a profile to block the os updates. As you said the update button was greyed out.
That was totally fine for me because it was necessary to block this feature.

But when i revoked the profile to update my devices the update was still greyed out.

I am using KCC on those devices because they were configured befor KC was released. The KCC
profile do not have the restriction to block os updates.

Non of my colleagues has ever sent any scripts to the devices.

The server has the version 14.2 Build 2894

Sorry. Yes my problem got solved too after i upgraded the MC agent to a newer version.

My goal wasn't to start the OS Update. i just wanted to re-activate the possibility to do OS

updates on the device.