StoRM Backend needs the database to be up before starting. It doesn't fail but the boot sequence could be better managed.
We can solve this by adding After=mariadb.service into the Unit:

[Unit]
Description=StoRM Backend service
After=mariadb.service

This means that if mariadb service is installed and enabled then it is required to be started before StoRM Backend. In case mariadb is not enabled, then this line has no impact.