This could very well be a timing issue. DejaClick normally tries to run the script as fast as possible on playback. If the script reaches an event in the script, and tries to interact with content that has not yet loaded on the page, this error can occur.
To help prevent this, you can try adding a Pause Interval to the script properties in DejaClick. Select the event that occurs prior to the error in the DejaClick sidebar and look for the Pause Interval property below. Adding a pause of 5000 ms adds a 5 second pause to the script. This may allow the page time to load before the script tries to interact with the content.
http://help.dejaclick.com/user-guide/firefox/details/properties-sidebar.html?skin=clean.nat%2cnat#D_j_Click_Properties_Sidebar_Con
