Oh I see! I was sure this was well known since it's quite easily replicable;
I create a solid box for ground then another one above (the character's collision box) on which I apply Platformer behavior, then I run, and I get this crash;
A crash inside a plugin's non-event code has been intercepted! This
may be a bug in the plugin, or a problem in Construct. The application
has exited. Available details of the location of the problem are below
(this may be approximate).
Object name: Movement_0_1
Instance: 1 (of 1)
It tells me I have the latest version. I'm sure it's something dumb...