Error when trying to change subscription owner (SSRS, SQL Server 2016)

I am using SSRS on SQL Server 2016 (native). All of our subscriptions are executed via a service account, so the owner of the subscription needs to be the service account, or the execution will fail. This means that, when creating a new subscription, the creator (or admin) needs to change the subscription owner after setting up the subscription.

I have a user who needs to be able to create subscriptions to reports. He is currently a member of the System User system role group, with membership of the Content Manager role group at root folder level (with inheritance). He can create the subscriptions in his name, but then gets the following error when trying to change the owner:

An error has occurred. The permissions granted to user 'domain\user.name' are insufficient for performing this operation.

My research thus far has suggested that Content Manager is the correct permission role to be able to do this. I have confirmed in the report server that all of the permission types are selected for the role, but this error is persisting. Please could someone advise if this activity must be undertaken by a System Admin (I don't really want to add him to admin, as he is a business user/analyst rather than a member of the data/reporting team)? Are there any other steps that I can try or settings that I can investigate to resolve this please?

Thank you.

Researched internet to check what role group was required for the type of change. Checked that the user was added to the Content Manager role group. Checked that the Content Manager role group had the relevant permissions selected. Expected user to be able to change the subscription owner, but error message returned.

I ran into this exact situation this morning. From my reading of the documentation, Content Manager should be able to do this. Whet I ended up doing was creating a custom system role (following this documentation: Create, delete, or modify a role (Management Studio)). It is essentially System User with the addition of "Manage report server security". It's dumb that I had to do this (they could create and delete subscriptions, but not change owners of existing subscriptions), but it worked.