I think I've found out why.
I'm guessing this forum uses downloadable fonts to display the relevant icons like 'Reply' etc. However, UK government security standards require browsers to block downloadable fonts because there are a large number of exploits that allow malicious attack via downloadable fonts.
Unfortunately, this site offers no alternative - it doesn't fall-back to a standard character, or a picture icon (eg .jpg). So the page just shows as white, with no button. It only shows when the mouse pointer goes over it and the ALT text appears.