lightdm or X server start very slow - takes a couple of minutes

Running xubuntu 18.04 LTS the boot animation does not work anylonger (instead I see a black screen) and it takes very long to boot. I changed /etc/default/grub the line #GRUB_CMDLINE_LINUX_DEFAULT="quiet splash" to #GRUB_CMDLINE_LINUX_DEFAULT="" and after a sudo update-grub I do see boot messages on the screen. At a certain point presumably X server or lightdm is starting and the screen blacks out as before. After 2-3 minutes the login screen appears normally.

I don't know what caused this problem, bug I think some update or configuration change by me, but I don't remember anything relevant. I've tried the properitary Nvidia driver and the default one. I use two monitors.

/var/log/lightdm/lightdm.log shows a huge gab of >2min between two lines:

...
[+0.94s] DEBUG: Session pid=1104: Authentication complete with return value 0: Success
[+0.94s] DEBUG: Seat seat0: Session authenticated, running command
[+0.94s] DEBUG: Launching process 1107: xubuntu-numlockx
[+1.03s] DEBUG: Process 1107 exited with return value 0
[+1.03s] DEBUG: Seat seat0: Exit status of xubuntu-numlockx: 0
[+1.03s] DEBUG: Session pid=1104: Running command /usr/lib/lightdm/lightdm-greeter-session /usr/sbin/lightdm-gtk-greeter
[+1.03s] DEBUG: Creating shared data directory /var/lib/lightdm-data/lightdm
[+1.03s] DEBUG: Session pid=1104: Logging to /var/log/lightdm/seat0-greeter.log
[+130.54s] DEBUG: Activating VT 7
[+130.54s] DEBUG: Activating login1 session c1
[+130.54s] DEBUG: Seat seat0 changes active session to c1
[+130.54s] DEBUG: Session c1 is already active
[+130.81s] DEBUG: Greeter connected version=1.26.0 api=1 resettable=false
[+131.18s] DEBUG: Greeter start authentication for peter
[+131.18s] DEBUG: Session pid=1585: Started with service 'lightdm', use
...

I've added allow-debugging = true to /etc/lightdm/lightdm-gtk-greeter.conf which adds some debug messages to /var/log/lightdm/seat0-greeter.log but AFAICS the messages start after the mysterious two minute sleep.

I couldn't figure out what happens between those two lines. Any idea how I could debug this further?

Edit: the problems disappeared, I don't know how.