Smartphone Crawl Errors in GWT (bbPress related)

In Google Webmaster Tools I get this message:

Dear webmaster of,
When your site was crawled by Googlebot for smartphones, we detected a significant increase in the number of URLs that return an HTTP 404 (page not found) errors. These errors may prevent Google from showing content on those pages in search results for smartphones. While it's not necessary to fix these issues, your smartphone users will be happier if they don't land on an error page.

Error pages are like this one:
(91725 or any number of course)

These are bbPress pages. "reponse" is the slug for "answer".
I don't think I changed anything about this recently.

Any idea?
Best Regards