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I have been having similar issues for a few weeks. I tried an old disc that I actually was able to successfully rip in the past, and the same problem has occurred. I tried the instruction below, but still no dice. Anyone able to successfully get 4k UHD discs to work? And if so what mkv version are y...

I also got a volume key unknown error on two discs I got (I re-ordered a second to make sure the disc wasn't just faulty). I submitted MKB20_v77_Barbie_4EAA for the second disc I received, which had the same hash as the first. Must be a new disc specific key in my area.