When trying to rebuild the cache I get the following error message:

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Internal Server Error

The server encountered an internal error or misconfiguration and was unable to complete your request.

Please contact the server administrator, xxxxxxx@xxxxx.com and inform them of the time the error occurred, and anything you might have done that may have caused the error.

More information about this error may be available in the server error log.
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I have looked in the server logs and haven't found anything usefull.  Is there something that I'm overlooking?

I was experiencing this same problem...but when I changed the path for the 
$serveDir variable while defining the configuration parameters to be the FULL
pathname starting at root the problem was fixed.