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Just test it, I was able to replicate this issues, but this is something Chromecast developers have to address not Sony.
I got few errors such as
Error loading media:
This may be caused by slow serveres or unsupported media types
No, because the same video that worked before (on lollipop) no longer works on marshmallow, it has nothing to do with Chromecast. I have a net speed of 14 Mbits (about 1.8 Mbytes) and my girlfriend has 30 Mbits (about 3.8 Mbytes) plenty of speed, so why does nothing work on marshmallow, neither "Full HD (60 fps)", nor "Full HD (30 fps)" nor even ordinary HD, the only one that does not stick and freeze is "VGA", this is unacceptable for such an expensive mobile, I paid Nkr 5500 which is serious money even here in Norway, really really pissed off about this!!!!!
Again, it seems that you didn't understood what I said, Chromecast its developed by Google, NOT Sony, hence Sony can't do anything about that app, as you pointed out, it worked fine with lollipop but not with marshmallow, that happen with smartphones, hence apps get updates all the time, not just to add options but to fix this bugs, what you are pointing out, its something that has to be addressed by Google/Chromecast's developers. Again, its a third party app, even if you had a Samsung, it would NOT be up to Samsung, nor LG, nor HTC, as it is not their app.