Question S3 Backup failed with no notification mail

[carini]

Server operating system version

Ubuntu 20.04.4 LTS

Plesk version and microupdate number

Plesk Obsidian 18.0.44.2

Hi, all

Recently I had a failed backup: on Plesk Backup Manager the following log is log shown

Home -> Tools & Settings -> Backup Manager
Backup Task June 15, 2022 04:17 AM Details
Error:
Unable to create the remote backup: Transport error: Extension transport: ext://s3-backup/server/: None

But no notification mail was sent

I found no relevant message in syslog: Backup starts at 4:15 CEST

Jun 15 04:15:01 plesk53 CRON[386557]: (root) CMD (/usr/sbin/logrotate /etc/logrotate.d/omi --state /var/opt/omi/log/omi-logrotate.status >/dev/null 2>&1) Jun 15 04:17:01 plesk53 CRON[388060]: (root) CMD ([ -x /opt/psa/admin/sbin/backupmng ] && /opt/psa/admin/sbin/backupmng >/dev/null 2>&1) Jun 15 04:17:01 plesk53 CRON[388062]: (root) CMD ( cd / && run-parts --report /etc/cron.hourly) Jun 15 04:17:38 plesk53 crontab[388533]: (root) LIST (*********) Jun 15 04:18:22 plesk53 python3[968]: 2022-06-15T02:18:22.728773Z INFO ExtHandler ExtHandler [HEARTBEAT] Agent WALinuxAgent-2.7.1.0 is running as the goal state agent [DEBUG HeartbeatCounter: 197;HeartbeatId: 75343134-0007-4C20-A4C0-0384FF D3041C;DroppedPackets: 0;UpdateGSErrors: 0;AutoUpdate: 1]

We have two other identical Plesk server, on the same network and on same S3 service (Scaleway) that worked perfectly yous hour before.

The problem is more "notification was not sent" than "backup failed"

Any hint?