in most cases, you do not need the .iso.
To download the image manually, follow these steps:
Have a look at the textfile updates.software-univention.de/d ... mg.xz.spec
It contains the names of all files which would be downloaded by the UCC-Images module. You can download them manually from updates.software-univention.de/d ... cc-images/ and place them on your UCS server in /var/lib/univention-client-boot/. After that, decompress the image .xz file and register the image:
unxz -v ucc-2.1-desktop-image.img.xz
chmod +x 95ucc-2.1-desktop-image.img.inst
cp 95ucc-2.1-desktop-image.img.inst /usr/lib/univention-install/
The image should now appear in the management system.