This is a yunohost core issue, have a look at this for a workaround:
This issue should be solved if your yunohost is up to date though : Helpers v2.1 : BACKUP_CORE_ONLY is not bind · Issue #2512 · YunoHost/issues · GitHub
This is a yunohost core issue, have a look at this for a workaround:
This issue should be solved if your yunohost is up to date though : Helpers v2.1 : BACKUP_CORE_ONLY is not bind · Issue #2512 · YunoHost/issues · GitHub