JS errors like this can occur from time to time. Usually you can get pat this problem by deleting the cache file for DW. Here is a link to a document that lists the steps to take for this:
common_installation_issues.pdf
If this does not work for you there may be an issue with the configuration folder. You can rename this to undo any corruption that may have occurred. Here is a link to a post with steps you can take to do this if need be.
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