Backup does not work for most apps

[DDvO]

When trying to backup all my apps and data, only around 25% of them (e.g., 21 of 97) allegedly got backed up correctly when Seedvault claimed "Backup finished". For most of the others, in the backup status an orange warning symbol is shown and the text "Waiting to back up.." and for few remaining ones a red failure symbol is shown without text.

This appears to indicate that SeedVault is of rather little use. I cannot even confirm that successful backup of the 25% actually worked fine, due to #682. Certainly for some apps the backup is incomplete - for instance, for the Signal and Photos apps success and a backup data size of 20 kB is reported, which is way too little!

Update: Part of the issue could be that when storage space runs out, the backup process wrongly claims "finished".