Great attitude 
There are three things that attract my attention:
- No space left on device, as you already noticed;
- Running off the SD-card;
- Failed signatures during update of repository
If the SD card is too big to be full already, combined with other errors, it could be the SD card is (on the way to be) broken. You might shut down the Yunohost and have it fsck’d on another computer (preferably Linux, I don’t know how well Windows and Mac handle ext4 these days).
The signatures could have to do with the Debian having a new stable version a couple of months back. I don’t remember it giving a signature error, but you could have a look at Updates may fail because "Repository [...] changed its 'Suite' value from 'stable' to 'oldstable'"