tag pages return error 404

Dear all,

This concerns my website http://www.rlcresearch.com.

I have a weird problem all of a sudden. I am not sure when it was introduced.

the tag-links at the bottom of my posts no longer work:
f.e. http://www.rlcresearch.com/tag/marceille/ returns error 404. On top of that, if I go in the ADMIN panel MANAGE/POSTS and click one of the tags behind the postnames there, it comes up empty as well.

I use the exact same theme and set-up for some of my other websites like http://www.rennessence.com, where it works perfectly.

Any suggestions? Thanks for your comments.
Raven