Docker OMV randomly stops working

Hello everyone,
I’m having issues with my Urbackup installation. It is randomly crashing after few hours or days, but on OMV dashboard, docker is “Installed and running” ; I have to restart it in order to wake it back up, which is a bit annoying. Does anyone have any idea about cause and how to solve this problem please ?
My installation is on OMV 5.5.23-1 on a RPi 4 2GB, and docker (version 5:20.10.3~3-0~debian-buster) is installed on the external hard drive which is also used for backups.
Thanks by advance for your support.