I'm just writing this down for public record, since there's no specific topic about this yet. I'm using Ubuntu Mate 18.04, Kernel 4.15.0-23-generic
Occasionally - perhaps once every 10 boot ups - it will freeze right before the login screen. It seems - though I can't be sure - that it happens after every update (whatever update).
On the whole, it isn't a big deal since there doesn't seem to be any error, and rebooting my pc sets itself straight but it is noticeable.