Widget panel asks “are you sure you want to do this…”

…and when I click “yes” no changes are saved.

The problem happens when I try to use a rebuilt version version of my theme. Widgets worked just fine on my test blog where I made the changes. Configuring Widgets in other themes still works. Could it be a conflict with previously saved settings?

I’m thinking about deleting all Widget-related data from the database, but maybe someone has a less crude solution. ??