[Sony Ericsson XPERIA X10 Mini] davie2000:
Thanks to your reply, I was able to root the device with flashtool. Afterwards I successfully started Superuser and switched off the phone again.

While unlocking the bootloader (button BLU) with flashtool afterwards it gave me the following output:

13/037/2013 16:37:33 - INFO - <- This level is successfully initialized
13/037/2013 16:37:33 - INFO - Flashtool Version 0.9.12.1 built on 2013-06-04 22:50:00
13/037/2013 16:37:38 - INFO - Device disconnected
13/038/2013 16:38:54 - INFO - Device connected with USB debugging off
13/038/2013 16:38:54 - INFO - For 2011 devices line, be sure you are not in MTP mode
13/038/2013 16:38:54 - INFO - Device connected with USB debugging on
13/038/2013 16:38:55 - INFO - Connected device : SonyEricson X10 Mini Pro
13/038/2013 16:38:55 - INFO - Installed version of busybox : N/A
13/038/2013 16:38:55 - INFO - Android version : 2.1-update1 / kernel version : 2.6.29 / Build number : 2.1.1.C.0.0
13/039/2013 16:39:02 - INFO - Decrypting E:\Program Files (x86)\[HARDWARE]\[HANDY]\Flashtool\custom\root\PsNeuter\psneuter.tar.uue.enc to E:\Program Files (x86)\[HARDWARE]\[HANDY]\Flashtool\custom\root\PsNeuter\psneuter.tar.uue
13/039/2013 16:39:10 - INFO - Pushing E:\Program Files (x86)\[HARDWARE]\[HANDY]\Flashtool\custom\root\PsNeuter\psneuter.tar.uue to /data/local/tmp
13/039/2013 16:39:10 - INFO - Pushing E:\Program Files (x86)\[HARDWARE]\[HANDY]\Flashtool\.\devices\busybox\1.20.2\busybox to /data/local/tmp/busybox
13/039/2013 16:39:10 - INFO - Pushing E:\Program Files (x86)\[HARDWARE]\[HANDY]\Flashtool\custom\root\subin\Superuser\su to /data/local/tmp/su
13/039/2013 16:39:11 - INFO - Pushing E:\Program Files (x86)\[HARDWARE]\[HANDY]\Flashtool\custom\root\subin\Superuser\Superuser.apk to /data/local/tmp/Superuser.apk
13/039/2013 16:39:11 - INFO - Running part1 of Root Exploit, please wait
13/039/2013 16:39:12 - INFO - Running rootit
13/039/2013 16:39:13 - INFO - Waiting for device. After 60secs, stop waiting will be forced
13/039/2013 16:39:13 - INFO - Device disconnected
13/039/2013 16:39:14 - INFO - Device connected with USB debugging on
13/039/2013 16:39:15 - INFO - Running part2 of Root Exploit
13/039/2013 16:39:15 - INFO - Running rootit2
13/039/2013 16:39:17 - INFO - Finished!.
13/039/2013 16:39:17 - INFO - Root should be available after reboot!
13/039/2013 16:39:17 - INFO - Device disconnected
13/040/2013 16:40:53 - INFO - Please connect your device into flashmode.
13/041/2013 16:41:58 - INFO - Device connected in flash mode
13/041/2013 16:41:58 - INFO - Opening device for R/W
13/041/2013 16:41:58 - INFO - Reading device information
13/041/2013 16:41:58 - INFO - Phone ready for flashmode operations.
13/041/2013 16:41:58 - INFO - Current device : U20i - CB511P8HKN - 1238-7168_R5B - 1235-3411_2.1.1.C.0.0 - WORLD-1-8_2.1.1.C.0.0
13/041/2013 16:41:58 - INFO - Processing loader.sin
13/041/2013 16:41:58 - INFO - Checking header
13/041/2013 16:41:58 - INFO - Flashing data
13/042/2013 16:42:00 - INFO - Loader : S1_Loader_Root_8e35 - Version : R4A045 / Boot version : R8A029 / Bootloader status : UNROOTABLE
13/042/2013 16:42:00 - INFO - Start Reading unit 00000851
13/042/2013 16:42:00 - INFO - Reading TA finished.
13/042/2013 16:42:00 - INFO - Ending flash session
13/042/2013 16:42:00 - INFO - Waiting for device to reboot
13/042/2013 16:42:00 - INFO - Waiting for device
13/042/2013 16:42:00 - INFO - Device connected in flash mode
13/042/2013 16:42:01 - INFO - Device disconnected
13/042/2013 16:42:03 - INFO - Device connected in flash mode
13/042/2013 16:42:22 - INFO - Device disconnected
13/042/2013 16:42:31 - INFO - Device connected with USB debugging off
13/042/2013 16:42:31 - INFO - For 2011 devices line, be sure you are not in MTP mode

There are no errors this time, but again nothing more happens: phone is loading and showing orange LED.

I am surprised of the log line saying that "Bootloader status is UNROOTABLE"?

btw: S1info still gives me a uppercase "R" for "not unlocked".

Can I proceed flashing the U20_2.6.32.61-nAa-06.ftf?