19.0 - Login-Screen - GUI-Login not possible

[cincinnatus]

Additional, possibly unrelated problem:
On booting, the system does not display the regular GRUB-menu (which allows to select Advanced options, Memory test etc.). So no chance to avoid the following problem by selecting an older kernel. Other than this, the system boots seemingly normal and goes to the regular login-screen.

Main problem:
In the login-screen, as always, I can select my user and type in my password. But upon receiving the password, instead of letting me in, the system blacks out the screen for a second and than goes back to the login-screen. This is the case both for my regular user (standard) as well as for my quasi-root (system administrator).

What I tried out:
This is not an issue of misspelled passwords. When I intentionally misspelled it, the login-screen replied something like "password not valid, please repeat" in yellow letters. (I am using the German-language-version, not sure how exactly this will be phrased in English.) It seems to me that this verifies that in the other instances the password was actually recognized by the system as correct.

Also, I went to command line via Ctrl+Alt+F4 and my username and password got accepted there without any difficulty. Oddly, however, on first entering this mode, behind "[computername] login:" there was displayed "^[[[D_" - I simply removed them via the backspace key, but it seems to me these characters should not have been there in the first place?

I tried the solutions suggested here: https://www.linuxslaves.com/2016/05/3-w ... -loop.html
ad 1) .Xauthority already had the proper owner.
ad 2) drwxrwxrwt read exactly as described, however, on using the next command, my quasi-root's password was not accepted for the sudo command.
ad 3) Same problem with the sudo command.

I am grateful for any ideas and pointers on how to log into my system via the regular GUI-mode again. And as well for any suggestions on how to alter my system to avoid such a problem in the future.


************************************
The system is a LinuxMint 19.0 Cinnamon, 64bit, running on a Thinkpad T510. This OS was installed only about two months ago (on a factory-fresh SSD). No other OSs are installed alongside.
The passwords were not changed recently (more precisely, they were not changed at all since installation).

[catweazel]

Additional, possibly unrelated problem:
On booting, the system does not display the regular GRUB-menu (which allows to select Advanced options, Memory test etc.). So no chance to avoid the following problem by selecting an older kernel. Other than this, the system boots seemingly normal and goes to the regular login-screen.

To try and get into GrUB you can hold down the shift key or escape key as soon as as the machine's power on, self test sequence has finished. Try that before trying to solve the bigger problem.

[rene]

Together with the password issue for sudo... if you use home directory encryption please see viewtopic.php?f=47&t=300064