Safari has an issue when working with Windows Authentication under IIS if Negotiate provider is enabled so AJAX POST is not working.
How to resolve?
In IIS, go to the Authentication settings of your website. Right click on Windows Authentication, choose providers and remove Negotiate, leaving NTLM this makes everything works fine.
[References]
AJAX POST Request Only Works Once in Safari 5
Negotiate Mechanism article in Wikipedia shows that it is not implemented in Safari
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