It was working earlier and all I have edited today was the lang_main file and changed case sensitive to CASE SENSITIVE in the registration part as some have been getting error registering.
I doubt this has caused any issues as its only a language-text edit, but now getting the 500 error
EDIT: Well it appears something went wrong on saving the lang_main.php as Iv just re uploaded the one from the original download file and replaced on my server and is working again.
Very strange
