I think there may be two issues here
Si and others have a problem if you click on their avatar within any thread they have posted on or even off the member page you get the 404 error and cannot read their profile. They can update their own profile but not much good if no-one can read it!!
Other times if one of us lucky people in the IT glitch group has a thread with multiple pages going when you click on the last page number it comes up with a 403 or 404 error if you click on the penultimate page it is OK. Looks like it is something to do with the end of a current page flipping over.
Have raised this again with James
Looking forward to seeing anyone going to the Mill this weekend, have my party frock ready