Same issue here - worked perfectly for a year, but for the last 1-2 months I can't watch much more than 5 minutes before it stops with the VC526 error.
Also on a Samsung TV (QE55Q95T). Virgin 1GB internet, with actual speed of 400mbps per Speedtest. No other heavy internet usage at the same time (just web browsing).
Only seems to affect BT Sport Ultimate. Normal BT Sport (non Ultimate) channels work fine.
I understand that the Ultimate is fairly intensive, but it's worked perfectly for over a year, and 400mbps seems like it should be capable? And regardless, I assumed the technology would auto-adapt the stream's quality based on capability to minimise need to buffer / stop.
The fact that it was so good when it worked makes it more painful!
