I upgraded last weekend, and my monitor (since long having run fine on 2560x1080_60.00) couldn´t get any higher 1920x1080. I tried adding a new resolution using xrandr but failed every time again.
A work around was returning to another kernel. Grub has available:
Found linux image: /boot/vmlinuz-4.13.0-16-generic
Found initrd image: /boot/initrd.img-4.13.0-16-generic
Found linux image: /boot/vmlinuz-4.10.0-37-generic
Found initrd image: /boot/initrd.img-4.10.0-37-generic
Found linux image: /boot/vmlinuz-4.4.0-97-generic
Found initrd image: /boot/initrd.img-4.4.0-97-generic
And returning to linux image: /boot/vmlinuz-4.4.0-97-generic helped me to get the higher resolution back into the settings menu.
With the newer kernel, it returned errors when trying to add modeline using xrandr.
I saw multiple questions on this, none of them (other than switching back to an older kernel) helped solve the problem.
Thanks and regards,
Bill
