I'll get the E502 page sporadically when using the site, i.e. i am already logged in. I usually refresh the page, sometimes a 2nd or 3rd time and it goes away - most of the time.
This occurs maybe once a month (or even less), so it hasn't bothered me enough to get FT support involved, which lately takes ages to respond to tickets anyway 

Never had it appear during login though.....

Your account has been temporarily locked. Please try again later.

Haven't experienced this, but this error message apparently comes from the FT software, not the server software (as I suppose the Error 502 does).

I called them, they say there is no problem at their end.  cleared my cache, still the same. Sent them an email a while ago, no response. Tried the UK link, no luck.

Well, it says your account is locked... I guess, it's safe to assume that it is... uhhmmm well, actually indeed locked. They can temporarily suspend accounts if there are e.g. copyright disputes or similar pending issues - from what I've heard, your portfolio stays online though and your images remain available for sale. Maybe your account was locked (suspended???) in error? 
Either way, you should keep on nagging them, since obviously the problem IS at their end.