Hi all.
I don’t know how to solve the troubles created by the use of the grub-customizer.
There are for sure several threads in the forum about that.
Maybe you could even consider restoring a previous snapshot from the assitant (although it could be risky, normally you try a snapshot fron the grub and restore afterwards if everything works).
As far as the default kernel to start with, I think you could give a look here: