My WAG is you're using WiFi somewhere between your browser & the NVR. For me, using WiFi is a crap shoot (sometimes it works & sometimes not) during firmware upgrades. My guess is it is a timing thing, meaning, NVR doesn't like any delays in receiving IP packets during the firmware upgrade process. By delays we're talking milliseconds here.
Solution for me was:
- switch to a "totally" wired connection between the browser & NVR.... when perfoming firmware upgrades, OR
- limit, as much as possible, any & all other WiFi activity (& keep your fingers crossed).