Hello there,
I must say I'm not a professional here, but I'd like to answer anyway, since I had exact same issue few days ago.
We're currently upgrading our MobiControl Cloud Instance from v. 14.1 to 15.2 in order to make sure we can manage Android 10 devices too, just like this Zebra TC21.

I think our issue here is Plugin Version, I solved by forcing installation of Plugin. 
Basically, on MobiControl, you should click on Device, then on three dots Button and find an Option called "Install Device Plugin".
After confirming, Agent on Device should restart and once device is online again you should be able to remote control onto device.

At least, this is how I think I solved, hope someone more expert than me will be more precise.

Good luck :)

Sergio

Hello Fotis,

Thank you for your post, were you able to see the same error when trying to remote control from the web (HTML5) as well as with legacy?

Regards,

Thank you Sergio, it worked.

Best regards