I don't know if my problem is the same as bmb, but on my case even the ssh cannot logon.
I always get timeouts when trying to interact with the router but as far as I can tell all the functionalities remain working.
Does this happen for both devices connected to WiFi and devices with wired connections?
What happens if you try to ping the router when the web interface stops working?
In a terminal (or cmd.exe in Windows), where
router-address is the IP address of your router:
ping router-address
You can Ctrl-C to stop. If you don't see a response immediately, that's a pretty bad sign.
If that worked, you can also try:
telnet router-address 22
You should see something like
Escape character is '^]'.
SSH-2.0-dropbear_2015.67
(Type Ctrl-], followed by Q, to exit)
If both work but you can't actually log in with ssh, try clearing jffs as that may have survived a reset.