if you can get to the lightdm log in interfac, the grahpics driver is ok
there are serval reasons of login loop:
dde is incomplete, you can install xfce4 in tty and see if you can get into xfce4
disk has no space

Yes the problem is dde...

Xsession_error

xsession_error:        error sending to systemd .... Startdde warning and panico message stop forma start display manager

Problem is multi monitor with nvidia property driver and dde

i met the problem often i install the NVIDIA driver in 15.3 version。 But  in 15.4 version ，have no problem。

so just use the Open source driver

What graphics card do you have?

Open source has lower performance than nvidia driver