My apologies for the recent failure of captchas, which apparently caused them to display with no text content. That's just awful.
Someone/something removed the fonts the captchas are using from the machine FOB is hosted on. (B) Unfortunately, rather than fall back in some sensible way, the captcha module just started failing and putting up textless captchas.
I've reinstalled the fonts, so captchas should work for now. I'll try to put together a fix to the captcha module to make it fall back better (in my copious spare time). Again, please accept my apologies.