Have you tried deleting the cache file to see if it resolves the issue for you? Here is a link to a thread that discusses removing this file to fix issues:
showthread.php?t=1589
If this does not work for you please post back and let us know the version of DW you are working with and the OS. There is a known issue with this tool in DW8 build 3453 that prevents the floater from loading correctly and will result in a similar error.