I had an unresponsive computer after a prolonged idle period (over night).
It occurred if there were many SPT wu's running at once, and it no longer happens after limiting to 1 at a time by dropping app_config.xml into the hidden project folder C:\ProgramData\BOINC\projects\boinc.termit.me_adsl.
app_config.xml content:

<app_config>

<project_max_concurrent>1</project_max_concurrent>

</app_config>

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Yes.
The easiest way is to limit the number of cores used.
Then you don't need to edit this file.

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You probably ran out of memory.

I made a small FAQ page https://boinc.termit.me/adsl/faq-en.php
It's in the main drop-down menu "Project".

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Yes, I didn't investigate, but it seemed like a memory issue.
I can confirm that lowering CPU % when not in use also solved the unresponsive computer over night problem. Lowering this setting will affect other projects that I run, but is still a more sensible choice.
Before that, I tested with a concurrent task count of 5 and all was good overnight.

Much appreciated FAQ page also.

Ok.
Fine!

All that remains is to find the right balance of resources for different projects.
This can be achieved through points 1. and 2. FAQ.

Since point 1 applies to a specific project (customized by the user in the web form of each project).
And point 2 operates in the boinc-client on a specific user’s computer.

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I believe its on the user, not the admin here. With such a large memory footprint of ~1.2GB per task you have 14.4GB of your 16GB of memory tied up if all 12 threads are running. Doubt thats enough left for Win11 let alone anything else. The user needs to start any investigation by saying what else was running at that time.

The user needs to start any investigation by saying what else was running at that time.

No investigation needed. It's likely a memory issue and limiting the number of SPT wu's running at a time, with either the config file or by lowering the CPU % setting, fixed the problem.