ERROR 404 in Communities

I get the following error for ANY wiki topic or subtopic:

ERROR 404 NOT FOUND
Oops. Fail. The page cannot be found.

All worked ok untill recently. No updates/changes/new plugins has been installed.
The very wiki topics do exist in the admin panel; the 'main' page with list of wikis does exist, but going further to other subtopics results in 404 error.

Please advise,

Thanks!