Comment 3 for bug 1827407

It's likely caused by the kernel. You should have some older kernel versions installed, but if not, you can install them via 'apt install linux-image-4.15.0-NN-generic' where NN is a number.

18.04 released with -20 (I think..), so pick something after that and 'bisect' these builds until you find the first version where it broke. Remember to select the correct kernel from the boot menu. It should be fairly easy to spot the offending commit in a specific kernel update.