After some research, we found a colleague modified "Limits" in "Default Web Site" like these screenshots:
He checked "Limit bandwidth usage" (this is NOT the problem) but he checked "Limit number of connections" and set low value (50 is too low for this application, there is more connections all day long)
Normally, there is NO limitation in "Default Web Site" as below screenshot:
Reinitializing parameter (uncheck "limit number of connections"), and now IIS is OK
Conclusion:
Does not modify IIS "Limit number of connections" if you don't have a real good reason (license reason for example) and only if you must know how many users use your application in same time.
You can use "Limit bandwidth usage" to limit bandwidth usage if you need to restrict Distribution Point bandwidth (reason:VPN, low bandwidth cable use by multiple application...)
