Failed software update of 'Welcome' package - 'Requires installation of untrusted packages' UM 16.04

[maro]

Continuing the discussion from 16.04 Ubuntu Mate Welcome untrusted sources:
Try this:

@lah7

I think the signing key is missing, in other words, the PPA wasn't added properly. Try manually deleting the PPA in Software & Updates under the Other Software tab.

Then, click on the subscribe to the updates button or add the PPA via the terminal:

sudo add-apt-repository ppa:ubuntu-mate-dev/welcome

It solved the welcome package problem on two of my machines.