Thank you too for reporting and reminding me.
The Bandwidth usage should handle 15 digits, so max with current code would be 999,999,999,999,999, i.e. 999 TB (not tested).
I don't know your data xfer rates so you have to calculate yourself "how long it can run...".
This could be extended if many need that.
Since you are one of very few having this problem , it's difficult to fix it. And time consuming.
But I can give you some tips on where to start looking:
When GUI problem:
Log out from router, Close all router tabs (all devices), open new tab and login again.
Never keep a router form/tab open until it times out.
Always go to router BASIC page when starting, never go directly to a sub-page.
Don't use the mobile app to connect to the router.
"top.location.href="/multi_login.html" comes from Netgear secret/not-open binary code uhttpd/net-cgi that I can not change or debug.
Maybe it checks if you are trying to login from more than one device without logging out the other one?
But you should think about yourself. What is different with your setup?
Long password (63 characters have been used by some and cause severe problems).
Password containing "strange" characters. The Netgear code is full of bugs concerning this.
You should try to not use any of these characters in the password: "Non-English-characters,$,!, <, >, @, %, *,?, .,+,-,/,|,\,&,=,',",`,´"
If it starts working then add "strange" characters one by one till you find out the "bad" one.
If nothing works for you, I suggest you just don't use that part of the add-on.
Or uninstall it, and check here now and then if someone come up with a solution.