This error looks like one you may receive if the cache file is bad. You should try deleting this cache file to see if it helps to stop this error. Here is a link to a thread that discusses deleting this file and the locations of where it is.
showthread.php?t=1589
Let us know if this does not solve your problem and also post back with the version of DW you are using including the build number, this info can be found under Help > About by clicking on the scrolling text.