If you look in FTP, there should be a file just called
error_log, or at least there is for me. Not that familiar with it. Anyway, just open that with a plain text editor and read what it says. There’s probably one in most folders, not quite sure how the errors are distributed to log files so it’s worth looking in root,
public_html or similar or in your wp plugins directory. Perhaps someone else can say more specifically? My guess is it would be in your public html folder (or whatever it’s called).
It’ll be organised by date, just look for the right time and any errors occuring in