Every overnight backup failed after the server rebooted

Polyclinique Ennour (a fictitious facility) backs up its patient records and its appointment database every night to bkp-en-01. This server has two network cards:

• eno1 (Intel, main network 10.30.0.0/24);
• enp2s0 (Realtek, backup network 192.168.50.0/24, to the storage array nas-en-01).

On Sunday at 04:00, automatic updates installed a new kernel and then the server rebooted. This morning, the report says “14 backup jobs failed: destination unreachable”. nas-en-01 is working (the other servers can see it).

Your mission: bring the backup interface back, without permanently reverting to the old kernel, and make sure the next kernel update does not break anything again.