Themes aren't changing for new sites

I have a multisite installation with 5 sites working.

Everything is working fine, but since last week, every new site I have created for tests purpose I can not change the theme.

I went to main panel and activated the theme for the new subsite.

On the subsite panel, theme management, I can see the "default theme" and the new one available to be activated.
When I clicked "activate" I can read the yellow alert "New Theme activated", but nothing change, the theme wasn't activate.

This happens with any theme in any new site installed (the old ones are working fine, I can change themes etc).

Weird.