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I've been learning how to develop on a Mac for about a year and have always been able to run a local server, for example, when developing in Ruby on Rails or Sinatra.

A few days ago I updated to Mountain Lion and I just tried to run a local server to start a Sinatra application. However, it fails with the below message:

== Sinatra/1.3.3 has taken the stage on 4567 for development with backup from Thin
>> Thin web server (v1.5.0 codename Knife)
>> Maximum connections set to 1024
>> Listening on 0.0.0.0:4567, CTRL+C to stop
/Users/me/.rvm/gems/ruby-1.9.2-p290@global/gems/eventmachine-1.0.0/lib/eventmachine.rb:526:in `start_tcp_server': no acceptor (port is in use or requires root privileges) (RuntimeError)

I've already checked that no other ports are in use.

What can I do to be able to run the server again?

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I changed your question to ask for the real problem instead of your attempted solution (see What is the XY problem?)‌​. "Permissions" is a very broad term here. What happens when you try to start your Rails server? Does that fail as well? –  slhck Dec 27 '12 at 6:31
    
How did you determine that your ports weren't in use? Did you use netstat command or something else? –  slm Dec 27 '12 at 6:32
    
@slhck I can start the rails server –  BrainLikeADullPencil Dec 27 '12 at 6:34
    
@slm I used netstat to confirm what seemed obvious (i had no servers running). –  BrainLikeADullPencil Dec 27 '12 at 6:35
    
Do you know offhand what port you're trying to use? I found this issue , github.com/mperham/sidekiq/issues/441. Says that this message may be a "thin error". –  slm Dec 27 '12 at 6:44
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