no login more after sleep (lightdm)

Hello Readers,

i have a very stange error. a debian9 (fresh install) whit XFCE shows
follow:

i start my laptop ( a X230): debian9 boot, login sreen, give: username
and pw. i'm in. on XFCE

Debian 9 goes to sleep, Debian wake up, i see my Login Sreen. I give my
Username / pw and: *no login* (more) ????

i change to an other tty, login works fine... switch back to GUI: login
sreen hangs up!

var/log/auth.log say's:

> systemd-logind[558]: Operation 'sleep' finished. lightdm:
> pam_unix(lightdm:auth): check pass; user unknown lightdm:
> pam_unix(lightdm:auth): authentication failure; logname= uid=0 euid=0
> tty=:1 ruser= rhost= lightdm: pam_unix(lightdm:auth): check pass;
> user unknown lightdm: pam_unix(lightdm:auth): authentication failure;
> logname= uid=0 euid=0 tty=:1 ruser= rhost= lightdm:
> pam_unix(lightdm:auth): check pass; user unknown lightdm:
> pam_unix(lightdm:auth): authentication failure; logname= uid=0 euid=0
> tty=:1 ruser= rhost=

any idea whats happend?

thx
Michael