[378.56.2] Merlin keeps asking for password, can't change any settings

Hi,

I am running 378.56.2 on an Asus AC66U. I wanted to reboot the router today and it wouldn't let me. When I open the web GUI, I am presented with the login screen. I login successfully and can browse around in the UI.

But as soon as I try to modify a setting, update the firmware or click the reboot button, the page refreshes and I am asked to login again. I login successfully but I am back on the homepage and nothing has happened.

To update the firmware, the firmware gets uploaded,...

[378.56.2] Merlin keeps asking for password, can't change any settings