After you have just written an image with Ubuntu Mate to a microSD card, a very small 5GB partition is created for the SYSTEM. After you boot the Raspberry for the 1st time from this card, the unattended upgrade is launched and it does not have enough free space on this partition to install updates and necessary programs. Then, when the Raspberry is booted from this card for the 2nd time, this partition automatically expands from 5GB to 64GB at the time of booting. That's why users can't figure out what the problem is.
To avoid this, after writing the image with Ubuntu Mate to a microSD card, before the first Raspberry boot, run GParted and increase the partition with the system to at least 8GB. This should be enough for the initial installation process.
I wrote about this in more detail in my blog.