After tree migration, I cannot add a new leaf. I get a javascript error: Cannot set property 'modWin

Hello all.

I migrated a tree from production to one of our test environments. The same tree in the test environment was replaced. Now we cannot add a new leaf in the desired spot. (We can add a new leaf in other spots, but not in the area that we want). Instead we get this error in a browser popup window: Updating DOM Unsuccessful: Error - TypeError: Cannot set property 'modWin' of undefined. This is obviously a javascript error, and I suspect that there's some missing or mangled data in the tree, but I can't figure it out. Any help? Thanks.