Posting the way Mark did here does need some knowledge of HTML and I think that is probably (I've not looked at the source) where things go wrong for you.
The symbols < and > play hell with HTML as they are opening and closing tags for HTML. I stand to be corrected on this but although the issue has been known for years, we at folkinfo are one of very few sites who have tried to find a means of allowing something to get through without having to have a user "pre-process" what is there - in that case, it is primitive - a check box at the end of the thread will set it to convert the symbol to HTML for you - a bit like the "auto line breaks here"
Be wary of the broken notation BTW. The Midi conversion at Concertina.net and other implementations of abc2midi break the standard for the midi. Unless the Midi has an explicit %%Ratio statement (I'd need to check on the documentation for exact details) the MIDI output is one where it is adapted to sound good for English Horpipes. It is not as written and can for example make a complete nonsense of a Strathspey. It's no problem though - as long as you know...