Available updates list collapsed

I have many Drupal sites and this problem only happens with one of them:

When I reach my "available updates" page, only the sections' titles is displayed: "Drupal Core", "Modules", "Themes"

The modules themselves appear for about one second and immediately, collapse to leave only the titles visible.

I'm using Switchtheme on this particular site and the problem is the same with each of the enabled themes, including themes provided with Drupal Core.

I tried to disabled Switchtheme and leave only one core theme enabled. It didn't change anything. Did the same with other modules which I thought could cause the problem...The problem was still there.

I have many Drupal sites, some of them on the same server and this is the only one on which this problem occurs. For some reason, I can not figure out what the cause is and, at the same time, I feel it must be something very basic...

Oh, and also...This doesn't happen with any other page. Modules list, themes list, etc...all appear as they should.

If anyone had an idea, it would be great...

Thanks!
Thierry