Comment 3 for bug 1470825

Great log file, thank you. Here's what happens:
...
12:43:42.951 Enter
12:43:42.952 TouchBegin (master, dev_id=2)
12:43:43.066 ButtonRelease (slave, dev_id=13)
12:43:43.067 TouchEnd (master, dev_id=2)
...

Two observations:
1) There are indeed touch events coming in, but only from the master pointer, none directly from the touch screen device 13. That's new, I haven't seen that happening before. Onboard sticks with mouse events (ButtonPress/ButtonRelease) in that case (meaning multi-touch won't be available).
2) The first ButtonPress event is missing, there's only a ButtonRelease. That's why there's no reaction to the first touch. When you use the mouse the initial ButtonPress event is there.

I've looked into 1) a bit. The screen seems to be USB connected and probably uses the hid-multitouch kernel driver and the evdev X driver. Support for the SYNA7500 seems relatively sensitive to the kernel version. This thread reports mixed results with various kernels and failures in particular with Vivid's kernel 3.19:
http://ubuntuforums.org/showthread.php?t=2187204&page=14&s=34cfa4caf159096f7cce745988b1ab59
If everything else fails, trying different kernels may make sense.

No idea what could cause 2) yet. I'm working on some way to reproduce it. Is there anything I should know about your setup? Is this plain KWin without any other bits of KDE?

Things you could try:
Set Preferences->Keyboard->Advanced->"Touch input" to "none". If there's no change (likely) set Preferences->Keyboard->Advanced->"Input event source" to "GTK".