Adhoc task failed: core\task\asynchronous_copy_task,Attempt to assign property "subquestions" on false

Hello  , 

In   Site Admin > Server > Task Logs   I have many messages  like that : 

Execute adhoc task: core\task\asynchronous_copy_task
Adhoc task id: 140471
Adhoc task custom data: {"backupid":"491479acbf367fd2d1826fbbcfdb14ad","restoreid":"7861493d7e1bc4eb9c02f71293ee2ac6"}
... started 12:45:18. Current memory use 38.5 Mo.
Course copy: Processing asynchronous course copy for course id: 748
Course copy: Backing up course, id: 748
... used 31856 dbqueries
... used 14.781877994537 seconds
Adhoc task failed: core\task\asynchronous_copy_task,Attempt to assign property "subquestions" on false

for couride 745 , 748 , and some others 

My courses are not duplicate with the reuse course option  & copy course  

My environment is the following:

Moodle 4.5.6 (Build: 20250811) / PHP 8.3.24 / MariaDB 11.4.8-MariaDB
SSH access available for PHP commands, but no root access.

Following the reading of various posts on these forums, I also tried disabling asynchronous adhoc tasks, but whether enabled or disabled, the result seems to be the same.
I increased the following PHP parameters:
max_execution_time = 720
max_input_time = 600
max_input_vars = 250000

I can't  even duplicate Quiz 

I was impacted with https://moodle.atlassian.net/browse/MDL-83541    but now I used successfully the fix script proposed but I still can't  have the copy backup&restore process achieve with success.

any thanks for any kind of help . 

Sam