Hi Paollo,
Yes, you are obviously not alone with the Xperia Backup & Restore application issues after the XZ1 last firmware upgrade, which also included the OS upgrade to Android 9.0.
I am but not sure if this problem is solely limited to Xperia XZ1 though, but it can be quite possible.
My experience with the official Sony Support staff (be it Slovenian, UK etc. -it doesn't matter which country they cover) is also more or less negative, but hey - they are smearing their own name there ... Most likely the fault is not on first level support but certainly is on every higher level by not providing or at least confirming (i.e. admitting) the evident new software problems encountered and analysed by their own customers.
So, today I managed to find a "one time" or "partial" or better say temporary solution to the problem of manually performing the backup via Backup & Restore application. This solution only lasts for one backup to your eventual SD card and for one (additional) backup to your online Google account. Any further attempt will again be unsuccessful and you will have to apply the same solution yet again when another backup is needed!
THE SOLUTION is quite simple and logical: go to Settings -> Apps & notifications -> See all {n} apps, select 'Show system' from the 3-dots menu in upper right corner, slide down to the 'Backup & restore' app, open its settings, select FORCE STOP, afterwards open its Storage menu, select CLEAR CACHE and CLEAR DATA as well(!), exit Storage menu, and finally select DISABLE (although this latest one step may even not be required, but you can try by yourself). That's all! A complete reset of the application! Next you go back into system Settings etc. and after allowing anew all the required access rights for your Xperia Backup & Restore app you should be able to perform one backup to the selected SD card and one backup to newly selected and signed-in Google account.
