Funny, my strong belief is that they (Verizon and Novatel) already know there's a problem with getting a successful update to these units as the apparent sole purpose of the update we have been trying to install is to allow the device to receive future updates.
As I said above, I am hoping that Verizon will release a downloadable file which we can use to force the update rather than trying it through the web interface.
I recently got the 4620L and also have this very problem with firmware update. According to update log, I have attempted update around 10 times in different areas, web browsers, devices, etc. And they all resulted in a 403 failure after power cycling really quickly. At least I know it isn't just my device now.
I stated this before and I still can't believe they aren't providing a manual update process with a app. They have one for the 4510. Mine actually stopped saying there was an update available for a day after I did a factory reset but its back saying update available(failing of course).