[@netweb Keymaster 12 years, 4 months ago]

Argh… I can’t reproduce this with either the TWenty Thirteen or Twenty Fourteen theme yet I see the issue on your site!

All of the following combinations work for me

Using a page with the [bbp-topic-index] shortcode

http://example.com/sample-page/ http://example.com/sample-page/page/2/
http://example.com/support/ http://example.com/support/page/2/
http://example.com/forums/ http://example.com/forums/page/2/

Using a page with the [bbp-forum-index] and [bbp-topic-index] shortcode

http://example.com/sample-page/ http://example.com/sample-page/page/2/
http://example.com/support/ http://example.com/support/page/2/
http://example.com/forums/ http://example.com/forums/page/2/

Not using a page (using native bbPress /forums slug with “Forum root should show – Topics by Freshness”)
http://example.com/forums/ http://example.com/forums/page/2/

Any change your using a modified Twenty Thirteen theme?

How about plugin conflicts?

Have you tried resetting/flushing your permalinks?