There was issue PPPM-11825 in Plesk Obsidian < 18.0.26.
If workarounds above do not help I recommend to enable verbose logging in /usr/local/psa/admin/conf/panel.ini
and in /usr/local/psa/admin/share/pmmcli/pmmcli-rc
and try to find errors in /var/log/plesk/PMM/backup-<date_time>/backup.log...