UPDATE: the error seems to be resolved. I was able to create an application.
The OP creates the application and sees this error in the web interface. The application is used to obtain the necessary OAuth credentials. Only after you have these credentials can you start using the API. Apparently, there is currently an error that is hampering the creation of applications. Twitter acknowledged the problem and is investigating: https://twittercommunity.com/t/does-not-allow-me-to-create-a-new-app/32528/9
The HTTP headers mentioned by DWRoelands are sent back when the API is actually used and exceeds the speed limits (as described in the document that it linked)
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