Finally got around to reporting this :) I love Cura, but have had to use version 4.6.1 for a while.
In any version since 4.6.1, most recently 4.8.0, I'm unable to slice anything.
The input fields in Shell settings for Top/Bottom Thickness, Top Thickness and Bottom Thickness are always red, unless I set the values to 0.
Even calculated settings are invalid.
Debian GNU/Linux bullseye/sid, Linux 5.10.0-4-amd64, Xfce 4.16, xserver-xorg-core 2:1.20.10-3
This shows the issue as well: