This error reproduced in Opera 10 in IFrame mode.
1. Click the Insert button - input control appears.
2. Uploading succeeded, everything is ok.
3. Click the Insert button again - browser displays "IFrame window not found!" message.
4. Click "Ok", click the Insert button again - input control appears, uploading is possible.
and so on...
ShowFrameBrowseButton can be true or false - no matter: "IFrame not found" appears anyway.