My machine just works because I don't mess with it except for updates/upgrades and I only visit a very narrow selection of destinations. No experimentation. Just making sure that all of Devuan's moving parts are in order and tended to.

Apologies if that offends you.

What a strange comment, why in the world would that offend me, or anyone for that matter? Did I offend you in some way with my comment? It wasn't targeted at you at all, it simply comes from a lifetime as an electrical and mechanical technician, and has to do with the importance of maintenance and not ignoring warning signs. Maintenance is always easier than repairs.

For all it's PITA times, .xsession-errors is still a good way to learn what's mis-behaving in your system, that's why I don't disable it, instead I fix the things it complains about.

This is how golinux does it.

Only after people with more clues provided the solutions. Zero credit due golinux on that one.

TLDR: If you don't care about .xsession-errors, bind-mount it to /dev/null. If you want to keep it (but still don't like huge files), set up logrotate with compression - even gzip will achieve near-magical compression ratios on a log file containing mostly repetitive messages.
If you want the underlying bug fixed, report it to the upstream for whatever GUI junk is flooding stderr.

FYI mate has become somewhat infamous to those of us who tinker with it, for spamming xsession-errors, almost anything can cause it, sometimes it just doesn't like a theme or a particular app. A few months back a version of caja was so bad that if you just hovered over the xsession-errors file it would start rapidly filling up with even more garbage at the rate of a mb every couple seconds. I filed on that bug and to the debian-mate team's credit they fixed it in 24 hours, but it was bad.

failed to turn on DPMS: DPMS is not enabled . I guess we found the issue , but this is an issue in of itself right ? I deleted the xsession error log , wich freed up the space , but what could be causing MATE power manager to spam errors ?

Certainly have found it two choices find out why the DPMS is not getting enabled to stop it spamming the log. I despise shoddy programming like that by the clueless fool that did it. Second option is this a bash/sh script you put in executable location to  allow cron to run it every two hour say to delete the old file and create new. First fin the permissions of the file.

zeus@9600k:~$ ls  -lh .xsession-errors 
-rw------- 1 zeus zeus 305M Mar 19 21:47 .xsession-errors

On my machine it is read write only so that is how you want it to be when recreating it. So something like this for the file.

#!/bin/bash

# A script to remove the spam filed .xsession-errors that has been filling my /home partition

rm /home/username/.xsession-errors
touch /home/username/.xsession-errors
chmod 0600 /home/username/.xsession-errors

Now that does it for the file save it to somewhere that is in your and set the executable bit for me I would use my /home/username/bin directory for things like this.

nano /home/username/bin/xsession_errors_remove.sh
chmod +x /home/username/bin/xsession_errors_remove.sh

Now set it up to run from the crontab by using crontab -e to add a line in there for the normal user. My listing showing a script I run ever two hours to backup my home directory.

zeus@9600k:~$ crontab -l
# Edit this file to introduce tasks to be run by cron.
# 
# Each task to run has to be defined through a single line
# indicating with different fields when the task will be run
# and what command to run for the task
snip ...
# 
# m h  dom mon dow   command

## Run my rsync snapshot script every two hours.
0 */2 * * * /home/zeus/bin/snapshot_home.sh  > /dev/null 2>&1

Change the path and file name to yours and you should have no more problems with that piece of junk messing you about.

https://hardinfo2.org/download

https://ibb.co/mr8vQ63n