can't access display manager or desktop environment when booting or installing from USB

This issue started with the 12/23 ISO and has persisted in subsequent ISOs (2/19, 3/20). The last ISO to work for me was the one from 6/19. Here is a comparison of my experience with these ISOs, as well as with an upgrade from Trisquel 7.

General notes:
- I test every ISO with both dd and Startup Disk Creator
- I have only tested with a USB 2.0 port[1]
- I am using a librebooted 64-bit Thinkpad X60

Upgrade from Trisquel 7:
- I installed T7 and upgraded to T8 last January
- running live via "try trisquel without installing": n/a
- after installing:
--- splash screen while booting same as T7 except that Trisquel logo has black background instead of transparent
--- same splash screen displayed while shutting down
--- no problems starting display manager or desktop environment

64-bit ISO from 6/19:
- my current install is from this ISO
- running live via "try trisquel without installing":
--- while booting I see a black screen that says "Trisquel 8.0" in the center with a row of 4 dots below
--- same thing while shutting down
--- I never see the usual splash screen with the Trisquel logo
--- no problems starting display manager or desktop enviornment
- after installing:
--- same behavior as when running live except that I only see "Trisquel 8.0" + four dots while powering down, not while booting

64-bit ISOs from 12/23, 2/19, and 3/20:
- running live via "try trisquel without installing":
--- splash screen while booting same as T7 except that Trisquel logo has black background instead of transparent
--- same splash screen displayed while shutting down
--- I never get the "Trisquel 8.0" + four dots
--- after booting:
----- I should see the display manager but instead get a black screen with a blinking cursor at top left
----- I can access a tty via Ctrl+Alt+F[1-6]
----- from there I can connect to the internet and update/upgrade
----- 'startx' gives me [2]
----- 'cat /etc/X11/default-display-manager' gives me '/usr/sbin/lightdm'
----- lightdm is already installed
----- 'sudo lightdm start' gives me [3]
----- I am able to install other display managers (gdm, xdm) and set them as my default while prompted, but I get the same behavior
----- Ctrl+Alt+F7 takes me back to the black screen
- after installing:
----- same behavior as when running live

[1] so the issue is probably unrelated to this: https://trisquel.info/en/forum/cant-boot-trisquel-8-usb#comment-129688

[2]
EE) (EE) Server terminated with error (1). Closing log file.e/trisquel/.local/share/xorg/Xorg.0.log" for additional information(
xinit: giving up
xinit: unable to connect to X server: Connection refused
xinit: server error

[3] ** (lightdm:2637) CRITICAL **: session_get_login1_session_id: assertion 'session!=NULL' failed

some possibly relevant threads:
https://trisquel.info/en/forum/cant-boot-trisquel-8-usb
https://trisquel.info/en/forum/trisquel-mini-black-screen
https://trisquel.info/en/forum/trouble

I just tried running live the ISO uploaded today (4/1). I used dd to burn the ISO, booted, and selected "Try Trisquel without installing." I got the same black screen with a cursor as with the 12/23, 2/19, and 3/20 ISOs. I have not tried installing the ISO yet, but expect the same behavior based on my experience with the previous three ISOs.

Thanks this is very useful information. I've been trying to replicate the problem but I have been unable so far, would you mind trying again the last one, with a change?

In the initial boot menu, after selecting the language, press f6 and then escape, so you can edit the boot parameters. Please delete all parameters other than "initrd=/casper/initrd boot=casper". If that works properly, then please add all other parameters one by one until it fails again.

Thanks again for helping with this issue.

Also if possible, it would be useful if you could post files /var/log/casper.log and /var/log/syslog. If you don't have access to those in a failed system, the output of dmesg would also help.

At what point should I be prompted to select a language? When I choose to boot to USB from the GRUB menu I am taken directly to the menu containing "Try Trisquel without installing". At this menu, F6 does not do anything. When I select "Try Trisquel..." I boot into the GUI-less system. If I press "e" I see a command containing "initrd=/casper/initrd boot=casper" but I do not understand the command well enough to know what is a parameter to try removing.

I'm having trouble attaching /var/log/casper.log and /var/log/syslog for some reason. I will send them to you via email.

I made new isos with the missing legacy intel driver, they should work now.

Running live works! I'll try installing later tonight.

Installation went smoothly. Everything looks great, with the exception of the blurred text in Abrowser[1], which I encountered within the first few minutes of usage.

Changing status of this bug to 'closed.' Thanks quidam!

[1] https://trisquel.info/en/issues/23853