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
Does not modify IIS "Limit number of connections" if you don't have a real good reason.