At the time I was told it's most likely an cache issue, I knew it wasn't as every other site I visit was working fine and it only happens on the forum at Area51, not the other Area51 pages.
Anyway the issue still remains 7 months later and I'm wondering if it is something to do with the top navbar tool tit,
Hover your mouse over the breadcrumbs in the top navbar and the tool tit is displayed incorrectly:

Hover your mouse over the breadcrumbs in the bottom navbar and the tool tit is displayed correctly:

Maybe it isn't the reason why every forum page I load on here shows a blank white screen for a second first but it shows there is something amiss in the top navbar which could possibly be affecting something else.