adb backup on Marshmallow - is it different?

Device and settings:

Goal: I wanted to take adb backup on Mint Linux (17.3) of my apps (with data if possible) and internal SD using this Full Backup of non-rooted devices as reference

Problems:

1. 
   

   When I key this in terminal

   
   
   

   adb backup -apk -shared -all -nosystem -f moto.ab

   
   

   I get the prompt "Now unlock your device and confirm the backup operation " in the terminal but nothing happens on the device- I do not see any panel to accept or enter desk top pass word as in pic below (it's the same if I try -system instead of -nosystem)

   
   

2. The terminal reverts to prompt for next command in a few seconds ( A 0 Kb file is created on the laptop)

Question

Am I doing something wrong or Marshmallow require some extras to take a backup as desired?

Moto devices are said to be "pure Android" so I expect this problem to be prevalent on Nexus devices running latest OS

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Edit: Things tried with varying sucesss

1. Nexus 5 Marshmellow ADB backup problems. Going by this post

(....) it appears that adb backup isn’t recognizing the -apk, -nosystem, and -noshared switches. Any idea why? From the command line, if i remove those switch the backup runs

adb backup -all -f moto.ab 

works in the sense that I get the option on panel to enter password back up (problem 1 above overcome) but the file size (105 Mb) shows all apps are not backed up (In comparison using ES File explorer, the size is 2 GB)

2. I had read somewhere on this site that Helium app is based on adb. Helium Wiki says

Helium is NOT available to Motorola devices. This is because Motorola phones have a bug that break Android's backup mechanism. Until this is fixed by Motorola, Helium will not work on those Androids (....) Update: Helium is working on the Motorola Droid Turbo.

That obviously explains why it doesn't work on my device was the logical conclusion. This called for experimenting, so I downloaded Chrome extension of Helium and activated it (it needed a little workaround explained here )

I selected all apps under Helium and it backed up (less some apps shown as "non backable") with a size of 110 Mb (similar to above experiment)

So, apparently, my device Moto X play is not the problem !

3. Tried solution here ADB backup creates 0-byte file; prompts for current backup password even though I never set one; “Failed to set password” for Desktop backup password recommending escaping spaces between arguments

adb backup -apk\ -shared\ -all\ -nosystem -f moto.ab and got a 0 Kb backup file