Assume you have tried to reload it via the GUI because if that fails it sounds like a bug that Zyxel need to address.
The backup file that my VMG8924.B10A produces has a .conf extension and I wonder if it's expecting that?
Unfortunately for the Rom there's no upload button in the gui.
There's upload button in the gui for firmware, wwan, and config.
I felt the tweak of my tail!
Unfortunately I don't have any experience with the VMG8623-T50B. (Is that one of the ZyXEL range which use a MediaTek SoC rather than Broadcom? )
Ah what a shame, I'm making some progress I think.
My googling seem to point to an issue with windows FTP not supporting passive mode, so through an Ubuntu terminal repeating the same process is now giving me Permission denied.
It's not clear whether I don't have permission to access the local file or the "remote" fw/rom-d.
I'll keep y'all posted if I figure it out.
Cheers