[djuggler]

I just now got this working! The solution is posted at https://bbpress.org/forums/topic/dropping-multiviews-support#post-16050

Quite simply, turn 1and1 hosting has MultiViews on by default and it must be turned off in .htaccess with:

Options -MultiViews

Then add the /bb-admin/rewrite-rules.php mod_rewrite AFTER the Options -MultiViews.

That was too simple to take this long to resolve.