Thank you for the answer!
Looking /etc/inittab, I found:
# Default runlevel. (Do not set to 0 or 6)
The runlevel is already configured as 3. The system starts in text-mode.
Looking again /var/log/dmesg, I found the line:
lxfb 0000:00:01.1: 121072 KB of video memory at 0xd0000000
When this kernel boot line is executed, the monitor blinks.
This is the exaclty momento that the system get locked if the VGA cable of the monitor is not attached.
If not attached, system get lock.
If attached, it continues the booting process.
I found this link after that:
"This is a graphics framebuffer driver for AMD Geode LX based processors".
There is a command 'fbset' to configure the video resolution.
Well, I think I need to disable this video mode, but I still don't know how to do it.