Apparently, at some point the syntax for using
adb backup changed and it’s not really well documented…
This is how I backed up my Android phone with
adb backup -f mybackup.bkp '-apk -obb -shared -all -system'
While before you would launch your backup with:
adb backup -f oldbackup.bkp -apk -obb -shared -all -system
Notice the lack of quotes in the old version.