Hello,

The #call returns a null by default is an expected behavior of CSP hyper event framework.

The main difference between sync (#server) and a-sync (#call) is that the 1st act like a DO command while the 2nd act like a JOB command. 

TBH, I don't see a good reason to get a "direct" response from the #call when the code is meant to be executed in the background. When there is a need to monitor progress of a job launched by a #call from a CSP page, the approach I always did was to have this (jobbed) code to update some global (with a %sessionId as a key, to avoid  parallel jobs to override each other) then have a timmed JS function in the page to #server another function on that page to monitor the global status (started, in progress which % done, finished etc.)

The Backup.Task is deliberately not registered as an SQL table, despite what the autogenerated documentation header says.

If you run the following SQL (in %SYS, using $system.SQL.shell) you get 0 results:
SELECT TABLE_SCHEMA, TABLE_NAME FROM INFORMATION_SCHEMA.TABLES WHERE TABLE_SCHEMA = 'Backup'

You can however work with this table in COS objects