If subscription contains the domain without hosting then subscription backup task fails with warning:
Warning: hosting "example.com"
Hosting on domain example.com is skipped from backup due to error: Can't use string ("") as a HASH ref while "strict refs" in use at /opt/psa/PMM/agents/PleskX/PleskX.pm line 4713.
The following error appears during the backup settings of subscriptions and domains without hosting. Such behavior is known as software issue # PPPM-3245 .