Log copy to USB stick doesn't work

[HugoPoi]

Usb stick with hypercube on it.
1Go free FAT32 filesystem

Log example

2020-05-15 21:56: [INFO] ===== Start HyperCube Service =====
2020-05-15 21:56: [INFO] Looking for HyperCube file
2020-05-15 21:56: [INFO] Detecting USB sticks...
2020-05-16 11:55: [INFO] HyperCube file found
2020-05-16 11:55: [INFO] Loading JSON
2020-05-16 11:55: [INFO] Extracting settings for Unix
2020-05-16 11:55: [INFO] Extracting settings for YunoHost
2020-05-16 11:55: [INFO] Extracting settings for Wifi Hotspot
2020-05-16 11:55: [INFO] Extracting settings for VPN Client (logging)
2020-05-16 11:55: [INFO] Extracting .cube file for VPN Client
2020-05-16 11:55: [INFO] Updating Debian root password
2020-05-16 11:55: [INFO] Changing hostname
2020-05-16 11:55: [INFO] Updating hosts file
2020-05-16 11:55: [INFO] Setting locales
2020-05-16 11:56: [INFO] Upgrading Debian/YunoHost...
2020-05-16 11:56: [INFO] Check online DynDNS domains list
2020-05-16 11:56: [INFO] Checking DynDNS domain availability
2020-05-16 11:56: [ERR] Unavailable DynDNS subdomain
2020-05-16 11:56: [ERR] Installation aborted
2020-05-16 11:56: [INFO] All logs have been copied to the USB stick - you can remove it
2020-05-16 11:56: [INFO] 4 hours (without reboot) before disabling this interface
2020-05-16 11:56: [INFO] Please, save this page with Ctrl+S

No logs file on the usb stick