Hi, would you kindly try clearing your cache and browser history. Also make sure you do not use any plugins that might be messing up your UI and loading of web components.
Next thing I would try is making sure i'm on latest firmware of the router and that I use a good, fast browser to display UI components.
It might sound weird, but you can also try connecting to WebUI through different connection. So let's say you used wireless to connect to router - then try using LAN connection and see if map loads normally.