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My guess is that you are running windows and your antivirus is blocking calibre. ESET in particular is known to create problems like you describe.
When the content server starts it tries to allocate a port. One test it makes is sending something to the port to check if there is already a listener. The problem antivirus programs will receive all such tests instead of returning an error to calibre, making it look like some program already has the port.
You can test this by starting your machine in safe mode with networking, then starting calibre. If it works, then something on your computer isn't playing nice.
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