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.