I'm going crazy to solve this problem : in our microzed Yocto distro, it is not possible to login using the usb cable (ttyPS1) but only via lan cable using ssh (with debug-tweaks option the login is root without password) . If I remove the ssh server from distro the ttyPS1 access becomes possible and I can login with the same root name without password !
I need the ttyPS1 access to monitor the eth0 situation that sometimes seems to freeze (no ping at all!) but if you try to login via ttyPS1 the eth0 communication restart ! It seems crazy but it's the truth.