(In reply to Christian K�nig from comment #1)
> Hi Andy,
> 
> sounds like the estimated bitstream buffer size doesn't fit into a 32bit
> integer any more.
> 
> Where can I download such a stream for testing?

I wondered that myself so I made one with ffmpeg/libx264.

Further testing shows that 4080x4080 works and >= 4082x4082 fails.

I tried getting libx264 to make constrained baseline but no difference.

These 2 were made like 

ffmpeg -i a-2160p60.mkv -vf scale=w=4080:h=4080 -c:v libx264 -vframes 10 
x264-4080.mkv

as they are only 10 frames tested like -

mpv --speed=0.1 -fs --hwdec=vdpau x264-4080.mkv

While here a shameless bump of a similar VCE issue :-)

I don't know the precise fail point for VCE but 3840x2160 fails 2560x1440
works.

https://bugs.freedesktop.org/show_bug.cgi?id=91281