I came across with this problem several times. Especially when I reinstall the codec package, Subtitle Workshop 2.51 becomes incapable of working with MP4 and MKV videos. I think there is not any clear explanation on how to solve this issue. This is a short and clear solution to fix both MKV and MP4 preview problem with Subtitle Workshop 2.51 :
For Win7 :
1- Install K-Lite package K-Lite Codec Package (Clear install)
2- Start>All Programs>K-Lite..>Configuration>ffdshow video decoder>codecs
set Raw video (at end of the list) to YV12
3- Start>All Programs>K-Lite..>Configuration>DirectVobS>General
set Loading to Do not load
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I found this thread via Google, and I must say it was of great help, saving me a lot of troubleshooting!
However, instead of installing K-Lite Codec Pack, I just installed ffdshow and Haalis Media Splitter. I also had to set Raw video to "All supported", as YV12 didn't work for me. Anyone following this tips, do observe that you should undo the settings afterwards, as it will degrade video performance.