I use XP/SP2 and my error was similar. Every attempt to convert using a version newer than 18.104.22.168 resulted in an abort.
There is clearly a .MKV problem. I have never had any issues with AVI files. This cursed MKV is a pox on the world of video.
I uninstalled Visual C++ Library and I have tried 22.214.171.124, with good results, but I am now getting pixelation with MKV files. Maybe subtle but occasionally there. Sort of scenes ripped partially or in whole. I also saw similar pixelation post using version 2 of ConvertX.
Again, I have moved back to 126.96.36.199 as a defensive play.
I think VSO will fix these isues. Also, they are not reproduceable, they just randomly happen. Almost like its timing oriented. So I now only have ConvertX running, and no other apps, when I do MKV.
Excellent detective work by VSO people!