How do I fix this http server error while previewing

I get the following error when previewing

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Preview HTTP server

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Unable to start HTTP server for preview. Construct 2 can try to reconfigure your computer to allow the server to run. The server is currently set to run on localhost The reconfiguration will be permanent and may affect other servers running on your computer (if any). Would you like Construct 2 to try and reconfigure your computer for preview?

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OK Cancel

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I am using C2 V264 on Windows 10 Pro V1809. This problem wasn't there two weeks ago and other than updating Windows I don't believe there have been any important changes to my computer.

Construct 2 asks to fix the problem. If I run with admin privileges it says

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Preview HTTP server

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The reconfiguration appears to have been successful. Construct 2 will try to restart the HTTP server when you click OK.

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OK

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But the problem isn't actually fixed.

For others with the same problem that find this in the future:

Ashley wrote a tutorial to help https://www.scirra.com/tutorials/279/troubleshooting-construct-2s-preview-server

I fixed it by changing the port. No idea why 50000 stopped working after years.