It appears that it's that bit of the URL that starts with "PHPSESSID" that's causing the problem, but I don't know how it's getting into the link.
From the log files it seemed that you're using Internet Explorer 11. So I fired up that browser on my computer and, lo and behold, I got the same error! But while I was investigating the problem, it suddenly went away. The PHPSESSID parameter disappeared and everything was working OK. As you say, MK, it's intermittent.