Default [Q] UbuntuDesktop: Install-or-mount - root but mounting not permitted

Hello,

I am trying to get UbuntuDesktop using Install or Mount from this forum runnning on my Samsung S4mini
http://forum.xda-developers.com/show....php?t=2627255

Anyhow always when I try to mount the linux.img it says: mount: permission denied (are you root?)
(please see the attached screenshot)

You probably first recommend me to check if I have my phone properly rooted and if I have the root privileges. Yes I have both.
For rooting I used this: http://forum.xda-developers.com/show....php?t=2364980
It worked successfully. The App RootChecker says, Phone successfully rooted, the App SuperSU also.
I installed Terminal Emulator for the Install or Mount and also BusyBox X+ for some missing commands. Both are in the list of SuperSU and when I am starting Terminal Emulator and typing "su" a message tells me that I have SuperUser rights.
Also the installation of Install or Mount was without any error - except at the end when it was trying to mount the linux.img.

I searched the forum and google for this problem - but all solutions are concerning problems when the users didn't type su or haven't properly rooted or something similar. Unfortunately I'm a new member and so not allowed to post my question in the Q&A of the Install or Mount. I hope it's ok to do it here now.
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