First: @jupe welcome to the forum!
And now your questions.
With kernel 5.12.x there were problems especially with suspend (see Need help with suspend under 5.12 kernel). One of my laptops had that problem all the time. With kernel 5.12.7 the problem was gone.
Does your laptop behaves like described in the thread above?
If not, what happens?
Can you please post following things:
To see which kernel is used at the moment:
uname -a
The last 20 entries before the crash. Please run this right after restarting your laptop:
journalctl -b -1 -r -n 20
No, your problem is not related to that bug in lightdm. This error message is annoying but not a real problem.