SB6190 Sneaky question...

I’ve had a Arris SB6190 paired with a Arris SBR-AC1990P router for about 5 years. Yes, this question is really about the router, but it definitely is triggered by the modem, and I didn’t see another appropriate place to post it.

In the last week, I haven’t been able to enter the admin page of the router. I can get to the login page at the default address, but when I hit the “enter” key on the keyboard (after adding username/pass) or click the mouse on the “Login” button, I get the error “Can’t open page because the server dropped the connection”. 

However, if I remove the the Enet cable on the WAN connection coming from the SB6190, and then access the router, I can login to the router just fine. I can make and save changes. But as one can imagine, it’s a hassle. 

I’ve tried rebooting (both devices), restoring factory defaults (both devices). The addresses of the two devices are the default addresses, subnet masking is set to the default.

Any ideas how to fix?

Thanks!