Hi!
Glad to hear it worked for you ;>}So I tried the trick someone mentioned here. I simply had MakeMKV backup without trying to decrypt. Once the non-decrypted copy was on my hard drive I pointed MakeMKV to that and had it create MKV from that. Amazingly it worked.
Please be advised that it's inappropriate to post questions regarding any commercial media that you don't own...Xmen-first class is going back to Netflix.