archiviosonoro.org/archivio-sonoro.html
archiviosonoro.org/archivio-sonoro/archivio-sonoro-dell-umbria.html
they are K2 Category menu item

this category has not problems:
archiviosonoro.org/archivio-sonoro/archivio-sonoro-della-basilicata.html
maybe because this doesn't use tags

Hello
please, can you help me to fix the problem? it makes useless the site
Regards

In your error_log which file is causing the issue?
Are you using any other K2 related extensions?

Thank you for the reply Krikor.
At this moment I can't tell you which file, the error log is about 345 Mb and I'm looking to understand how to open it (Textwrangler on Mac doesn't).
Anyway, I disabled the only K2 plugin I use (SW K2 Map from StyleWare) and yes, maybe it is the problem because now I can see all the categories without errors. I just contacted StyleWare support to submit the problem.
I'm sorry, due the problem was the same of this thread I supposed it was on K2. I will update this asap.
Thanks

Let me know if that was the issue so I can keep an eye out.
As for your error_log. a) It's a bit too largem so try correcting some issues, b) Look for Fatal Errors.