It looks like you installed:
- Raspbian FULL,
using Matt's webs page to customise the partition sizes.
The first error message is due to my mistake.
I have just updated retropie to v4.5.1 and inadvertently changed its name from retropie2 to retropie3. Matt's webpage can therefore no longer find the original files and errors out. Apologies. I have just restored the old retropie2, so it should work again, although you might have to wait up to 24 hours for Matt's webpage to refresh its data. In the meantime, you could try a couple of things:
1. exclude retropie from the backup
2. Edit recovery.cmdline and remove the 'alt_image_source="...." option
3. Try only selecting 1 OS at a time to backup, to avoid errors in one OS preventing you backing up the others. Then post what is still problematic.
It should be possible to backup raspbian_full, so I'm not sure why it claims an update is required. Maybe that is due to the first error, or it could just be due to the version of Raspbian you installed - what version/date do you have installed? If removing alt_image_source does not fix it, let me know and we will fix it another way.
It will not be possible to backup Xbian using PINN because it uses a BTRFS file system. You will have to do this manually, I'm afraid.
Sorry for the unexpected inconvenience.