Same problem here with latest firmware. I first noticed it a few weeks ago. If I reboot the router I can access the WebUI for a few hours and then its back to just the swirl in the tab of the browser and then eventually times out. I just run the router.. no mesh devices etc. Seems to be a memory leak?
ASUSwrt (WebGUI) down (first time) AXE16000 126.96.36.199.388.23012 both via WiFi and Ethernet (through switches) IP entered manually and asusrouter.com .
Access through smartphone app (both Android and iOS) seems unaffected.