When I converted from classic two years ago. Bookmarks and indexes were issues even with the redirectors in place.
This is the point, the redirectors work from old URI to new URI, but they don't disallow concurrent URI methods.
To make it worse for me I went from a windows host to linux. So Case issues with URls came into play.
That's a 404 issue for an htaccess nightmare !
...
Well, one of my sitemap threads talks about the ubb multi urls
Yay! So where do I find that thread - I've looked...?!!? (Well, a bit...)
