I don't know. You can try the nuclear option (this will wipe out any customizations you have in the Web interface); it fixed an similar issue for someone else.
I'm not a huge fan of it, but it may get you up and running since we don't really know what the problem is.
sudo rm -rf /var/www/html/admin
sudo git clone https://github.com/pi-hole/AdminLTE.git /var/www/html/admin