[Mach30SiR]

From TaylnOne over at XDA:

Sprint HTC Mogul/SERO Notes

It seems Sprint is up to shenanigans in regards to tethering for SERO users. Those of you running Sprint ROMs should navigate to the following registry key "HKEY_LOCAL_MACHINE\Comm\InternetSharing" and delete the string value named "Extension". This will allow you to connect to the Internet via the Internet Sharing app without the dreaded error 67/authentication errors. Previous to this fix, Mogul SERO users would have to connect to the Internet via another method (such as starting Internet Explorer first) before pressing the Connect button in the the Internet Sharing application.

If you're really paranoid (it has been concluded these entries have no meaning when using Internet Sharing), using a registry editor, navigate to the "HKEY_LOCAL_MACHINE\Software\OEM\WModem" key, and set the "CheckProfile", "Multi-NAI" and "WModemDUN" DWORDS to a decimal value of "0".