I have the following last line in the error.log after a crash
[18:28:53][E][pdx_gui_container.cpp:142]: gui/window_county_view.gui:1824 - A (flow)container ({}:{}) can't have an hbox/vbox ({}:{}) as a direct child.
So I guess the crash is caused by that.

Naturally, I searched in my file if there was any box as direct child of a container but couldn't find any.

For those wanting to help here a pastebin for the file
https://pastebin.com/MiFSLRLb

More context:
I'm trying to update the mod "Balanced Buyable Building Slots updated" for the new update.
For that I took the whole vanilla file and just updated the "bottom right" section with the modification (Duchy/Special buiolding on one ligne, new button to buy slots, etc.)

And I'm kinda stuck with it for a few hours now.