It looks like an HTTPS request using roPosterScreen, roSlideshow, etc. is only negotiated by the Roku to use TLS 1.0, whereas an HTTP request using roUrlTransfer negotiates the use of TLS 1.2. Due to security vulnerabilities discovered in older TLS versions, it's possible that some servers are refusing to allow connections that use the older versions, which might explain why one CDN (Akamai) works and the other one (Instagram) does not.
It would be helpful if someone from Roku could look into this and verify whether roPosterScreen, etc. erroneously only supports TLS 1.0.