Unable to add ringtones via iTunes

[TimeTraveler7]

As it turns out, I can't add ringtones to my iPhone anymore with iTunes. If I go to "On My Device / Tones", there's a list of all previously imported ringtones on the right. However, when I try to drag&drop a new one (either to the right side, or to the toolbar/Tones on the left), I get the dialog box "The ringtone "..." was not copied to the iPad "..." because its duration is too long."

The ringtones are way below the 30-second threshold. Any idea what could be wrong? Is there any other way of importing them?

I have the latest iTunes running on 64-bit Windows 7, and the iOS devices are also on the latest version.