403 Forbidden Error with Dynamic DNS

> I get a 403 Forbidden error (a red frame with just the line "403
> Forbidden" when I try to enter Dynamic DNS settings in the AC2400.
> [...]

   Sounds to me like a firmware problem.  I'd start with a settings
reset (and manual reconfiguration).  I wouldn't bet much on it, but it's
worth trying.

   Visit http://netgear.com/support , put in your model number, and look
for Documentation.  Get the User Manual.  Look for "Factory Settings".

> Firmware is V1.2.0.42_1.0.1. [...]

   If the settings reset doesn't help, then you might try a different
(older) firmware version.

   Visit http://netgear.com/support , put in your model number, and look
for Downloads.  (For older versions, under Firmware and Software
Downloads, look for "View Previous Versions".)  Find the kit(s).
Download the kit(s) you want.  Read the "Release Notes" file for
instructions.

> [...] The error appears so quickly that it doesn't even seem like it's
> gotten a response from an external site. [...]

   You're talking to your router's own management web site, so that's
not amazing.

   On my D7000, for many different firmware versions, "BASIC >
ReadySHARE : Safely Remove USB device" produced a "403 Forbidden" error
page.  I seldom use that feature, so I can't easily say when it
happened, but it did eventually get mended.  It's not as if Netgear
actually tests this stuff before it's released.