Hi Andre,
Have you tried pulling latest image as you might know that 'manifest unknown' generally means requested image or tag is not correct/not available. So, if you don't see the required image version at product repo then it is not released yet. What's the urgent need of 11.1.5 image, may be if you can provide valid justification to product support team by raising a ticket to them, they may provide you appropriate answer.
Until IBM pushes the latest image versions, I think you won't get it anywhere apart from official product repo.