error in the recent list posts, after clicking on a tag

Hi,
i have a lateral column with recent post’s list.

I try to explain:
– I open one of the post
– I click on a tag in the TAGGED list at bottom of the post
– lateral column (recent post’s list) reports some strange list: posts with numer, without titles, mixed with other posts. If I try to click on one of these numbers, the result is “Error 404. The page You requested was not found. Perhaps searching can help.”

Can you help me, please?

Thanks,
Marco (IT)