Incompatibility between ACPI and the X-Windows server

Hello,

I'm trying to update a notebook, to run 5.2. The beast is an oldish Vaio 
PC-F707 (Pent-III + Neomagic).

When booting with ACPI enabled, X-Windows just stays "frozen" : the screen is 
blanked, then there is no more progress (and no log in /var/log/XFree.log).
When booting without ACPI, X-Windows runs happily.

How do I help debugging this problem ? When the screen is frozen, I can't 
escape to the kernel debugger.

	TfH

PS : the machine also runs flawlessly with a recent 4.9-Stable and the 
straight X-4.3.0 server. As the machine used to run 4.x, I do not know when 
the X11 server stopped running alongside with ACPI

PS2 : a successful X11 start log (under 5.2, without ACPI) is accessible at
http://thierry.herbelot.free.fr/XFree86.0.log

PS3 : an ACPI dump is accessible at http://thierry.herbelot.free.fr/foo.asl