I based my version from 5.0.0. There are a lot of changes in later versions I guess, but those versions don't really work well. You would have to go to github and read each commit one by one after 5.0.0 to see what was implemented/fixed. They are a lot:
https://github.com/mjuhasz/BDSup2Sub/commits/master
I tried to fix 5.1.2. I really tried. But it was so messy with so many errors that I picked the last version that worked (5.0.0) and started from there.
Also I'm wondering what's the use you guys have for this software. Nowadays you can mux a SUP file into an MKV container, so you don't really need to convert SUP to anything else. In my specific use case, I want my MKVs untouched for several reasons and the subtitles external. SUP files can't be external files, they only work when they are inside of a container like MKV. But if I wouldn't mind to modify my MKVs, I would just put the SUPs inside of them, and there wouldn't be a need for BDSup2Sub.