Is this a js type of error or is that the entire error message you are getting? If that is the entire error message I have seen this happen before where DW had a problem with the connection and could not work it out. If this is the case there are a couple of things that you can try to help with this:
Recreating the site cache
Go to Site > Advanced > Recreate Site Cache
Resetting the workspace layout
Window > Workspace Layout > Reset
If these do not resolve the problem you can try deleting the cache file with the instructions here:
If the problem persists after this there might be a problem with the connection and you may need to remove it from the site and add it again as a new connection. Please post back and let us know how it works for you.