Hi Roberto,
Can you please check whether authorisation of the task assigned to user is 'Inherited from the project/phase' or it is empty.
It should be empty, since write access is provided exclusively to the task through staffing.
Sometimes this issue may be due to HR personnel number & user id linkage to business partner is wrong.
Check whether the user accessing the task is correctly linked to the business partner staffed.
Regards,
Ravi
