Is it possible that it could for some reason be a favicon issue? Checking the access-logs I can see this repeated over and over in thousands of lines with a "404" error saying:
IP - - [20/Jun/2011:06:57:12 -0400] "GET /favicon.ico HTTP/1.1" 404 - "-" "Mozilla/5.0 (Windows NT 6.1; WOW64) AppleWebKit/534.30 (KHTML, like Gecko) Chrome/12.0.742.100 Safari/534.30"
^ With different codes each time.
I doubt it's a fave icon issue. I had noticed this happening on the site in question yesterday. You could always try adding a favicon, though that shouldn't be causing the error document to display.
It could be the favicon, however it's unlikely for a favicon to cause such a persistant problem, because most browsers ask for the favicon once and then not anytime soon again. So as soon as the user goes on to the next page in your forum the error is gone. Of course if you see this location mainly with guests, it's possible...
Even with disabling the 404 Error it shows "Unknown Location" as opposed to "Seeing an Error Page", and it still doesn't seem to change after following the MyBB 1.4 issue when this was around. So far adding a favicon, disabling the 404 Google SEO errors + trying the old technique don't seem to be working.
Did you also remove the ErrorDocument directive in .htaccess, because that's what makes the requests go to misc.php?google_seo_error=404 in the first place
Ah, yeah. I deactivated the Google SEO 404 function and modified the .htaccess to remove the ErrorDocument from it, as well as changed the functions_online.php accordingly. The issue seems to be resolved now, thanks, Frostschutz.