A while back, while doing a server upgrade, the spell check that's built into the forums apparently got broken and stopped working.
Because I don't use it, I didn't know it was broken until a couple of days ago, when I was testing some stuff and noticed that it didn't work. When the spell check button was clicked, it opened up a blank window.
So it's fixed now and working as intended, for those of you who do use it. It's probably not a major issue, since web browsers now come with built-in spell check, but I hate when things don't work.
Thanks for your patience.