Don't know whether it's really relevant to other's issue. Posting here in case it is useful to anyone:
I had similar issues with hibernation: menu bar unresponsive, couldn't launch applications via GUI (but could via terminal). Found out journalctl was riddled with GPU errors. A user (Nycticorax) pointed me to a solution (https://discuss.getsol.us/d/3052-nvidia-hibernation):
Basically, the partition listed in /etc/kernel/cmdline.d/10_resume.conf wasn't my swap partition (I'm using LVM so it seems it has to be a /dev/* file (which can be found with swapon -s).

ah ! ok, it indeed does happen only once per boot (if I understood you well), and indeed without any further problem that I'm aware of anyway. thx for your reply

Still getting this error on each boot:
Nov 10 16:11:47 alecto nvidia-persistenced[814]: Failed to query NVIDIA devices. Please ensure that the NVIDIA device files (/dev/nvidia*) exist, and that user 143 has read and write permissions for those files.

I'm really sorry ? ? I just realised I provided the wrong source: It should have been https://www.x.org/releases/individual/data/xkeyboard-config/xkeyboard-config-2.28.tar.gz . And therefore update to 2.28 would be a more appropriate request… (corrected)