[Sony Xperia P, U, Sola, Go] blavov:
Well, I tried it now using the linux version of the flashtool (command line) - and it doesn't work either. ;-(

I can connect the phone, and it is detected on the USB bus (and the lightbar turns green):

Bus 001 Device 006: ID 0fce:adde Sony Ericsson Mobile Communications AB Boot loader

But when I try to flash something, the tool doesn't even start:

23/013/2016 15:13:36 - INFO - <- This level is successfully initialized
23/013/2016 15:13:37 - INFO - Flashtool Version 0.9.19.8 built on 24-09-2015 22:00:00
23/013/2016 15:13:37 - INFO - Choosed /home/kasi.mir/flashtool/ST25i_6.1.1.B.1.100_Sony.ftf
23/013/2016 15:13:37 - INFO - Preparing files for flashing
23/013/2016 15:13:37 - INFO - Preparing files for flashing
23/013/2016 15:13:37 - INFO - Please connect your phone in flash mode
23/013/2016 15:13:38 - INFO - Opening device for R/W
23/013/2016 15:13:38 - INFO - Device ready for R/W.
23/013/2016 15:13:38 - INFO - Reading device information
libusb_bulk_transfer (read) : I/O Errors
java.lang.NullPointerException
at java.io.ByteArrayInputStream.<init>(ByteArrayInputStream.java:106)
at flashsystem.S1Packet.addData(S1Packet.java:133)
at flashsystem.S1Packet.<init>(S1Packet.java:33)
at flashsystem.io.USBFlashLinux.linuxReadS1Reply(USBFlashLinux.java:54)
at flashsystem.io.USBFlash.readS1Reply(USBFlash.java:51)
at flashsystem.X10flash.openDevice(X10flash.java:997)
at flashsystem.FlasherConsole.doFlash(FlasherConsole.java:191)
at gui.Main.processConsole(Main.java:84)
at gui.Main.main(Main.java:38)
23/013/2016 15:13:38 - INFO - Unable to read from phone after having opened it.
23/013/2016 15:13:38 - INFO - trying to continue anyway
libusb_bulk_transfer (write) : I/O Errors
Error : did not write all data
libusb_bulk_transfer (write) : I/O Errors
libusb_bulk_transfer (write) : I/O Errors
Error : did not write all data
libusb_bulk_transfer (read) : I/O Errors
java.lang.NullPointerException
at java.io.ByteArrayInputStream.<init>(ByteArrayInputStream.java:106)
at flashsystem.S1Packet.addData(S1Packet.java:133)
at flashsystem.S1Packet.<init>(S1Packet.java:33)
at flashsystem.io.USBFlashLinux.linuxReadS1Reply(USBFlashLinux.java:54)
at flashsystem.io.USBFlash.readS1Reply(USBFlash.java:51)
at flashsystem.io.USBFlash.writeS1(USBFlash.java:34)
at flashsystem.Command.writeCommand(Command.java:111)
at flashsystem.Command.send(Command.java:126)
at flashsystem.X10flash.hookDevice(X10flash.java:974)
at flashsystem.X10flash.openDevice(X10flash.java:1011)
at flashsystem.FlasherConsole.doFlash(FlasherConsole.java:191)
at gui.Main.processConsole(Main.java:84)
at gui.Main.main(Main.java:38)
23/013/2016 15:13:38 - INFO - Start Flashing
23/013/2016 15:13:38 - INFO - Processing loader.sin
23/013/2016 15:13:38 - INFO - Checking header
libusb_bulk_transfer (write) : I/O Errors
Error : did not write all data
libusb_bulk_transfer (write) : I/O Errors
Error : did not write all data
libusb_bulk_transfer (write) : I/O Errors
Error : did not write all data
libusb_bulk_transfer (read) : I/O Errors
23/013/2016 15:13:38 - ERROR - Processing of loader.sin finished with errors.
java.lang.NullPointerException
at java.io.ByteArrayInputStream.<init>(ByteArrayInputStream.java:106)
at flashsystem.S1Packet.addData(S1Packet.java:133)
at flashsystem.S1Packet.<init>(S1Packet.java:33)
at flashsystem.io.USBFlashLinux.linuxReadS1Reply(USBFlashLinux.java:54)
at flashsystem.io.USBFlash.readS1Reply(USBFlash.java:51)
at flashsystem.io.USBFlash.writeS1(USBFlash.java:34)
at flashsystem.Command.writeCommand(Command.java:111)
at flashsystem.Command.send(Command.java:126)
at flashsystem.X10flash.processHeader(X10flash.java:303)
at flashsystem.X10flash.uploadImage(X10flash.java:320)
at flashsystem.X10flash.sendLoader(X10flash.java:400)
at flashsystem.X10flash.flashDevice(X10flash.java:749)
at flashsystem.FlasherConsole.doFlash(FlasherConsole.java:192)
at gui.Main.processConsole(Main.java:84)
at gui.Main.main(Main.java:38)
flashsystem.X10FlashException
at flashsystem.X10flash.uploadImage(X10flash.java:340)
at flashsystem.X10flash.sendLoader(X10flash.java:400)
at flashsystem.X10flash.flashDevice(X10flash.java:749)
at flashsystem.FlasherConsole.doFlash(FlasherConsole.java:192)
at gui.Main.processConsole(Main.java:84)
at gui.Main.main(Main.java:38)
23/013/2016 15:13:38 - INFO - Ending flash session
libusb_bulk_transfer (write) : I/O Errors
Error : did not write all data
libusb_bulk_transfer (write) : I/O Errors
libusb_bulk_transfer (write) : I/O Errors
Error : did not write all data
libusb_bulk_transfer (read) : I/O Errors
23/013/2016 15:13:38 - ERROR -
23/013/2016 15:13:38 - ERROR - Error flashing. Aborted

What is the problem here? Why can't I even read ID information? :(