I reproduced this use case with a Out-of-box report "Service Portal KB Article Views" which Report admin was not able to share.
Then i realized that the user also needs a sp_admin access to share this report.
My conclusion: Adding itil role should do the trickfor all platform applications. Exception to it is only Scoped Applications reports ( like HR) which needs specific Aplication level access to Reporting admin.
Hope it all makes sense!!
