The problem seems to be when an "&" is included in the page name instead of in the query.
I reinstalled the software from scratch before trying this again, just
in case it was anything I had done, so this is, IMHO, a bug.
My guess
is it's the web.config, which again seems not to be affected by the SEO
Setting - mine is disabled but it still seems to use the web.config since renaming web.config causes a total crash.
If when I get the 403, I change the url to /forum/forum1.html?OB=asc instead of /forum/forum1&OB=asc.html then it works.