I've already been through the rigmarole with the Azure team on this, who tell me I need a 'paid' Azure subscription for it to be investigated.. but here goes.
As we do with EVERY client, we got round to customising our own internal 365 tenant's logo/custom branding in the Azure AD Admin portal.
To clarify I'm in;
365 Admin Center > Azure AD Portal > Azure Active Directory > 'Company Branding'.
So far, so good. Click on the 'Default' entry and attempt to upload images, 1920x1080px - Yup. <300kb - Yup.
Then comes the error....
I cannot replicate this issue on ANY other 365 tenant. I am signed in as a global admin with every role possible, I've got an Azure AD P1 license attached to this user and I'm using the same set of images for the sign-in page background than I did on the other 3 tenants I've tried. Never seen this before, I can't replicate it on a brand new tenant with exactly the same licensing/setup, therefore I'm left with it being a 365/platform issue, but that's it... This should be a 30 second job but is turning into a massive pain.