Lubuntu 15.10 dual monitor setup issues

**Problem description** I recently updated my two monitor system from Lubuntu 14.04 to Lubuntu 15.10. I set everything on the new system exactly as on the old one before and I figured out a strange behavior on my desktop: The primary monitor 1, the start menu position and the position where the desktop icons are located somehow were messed up. Additionally I could not set desktop wallpaper for both monitors. On the screenshot below you could see the situation. Not only the position of the start menu and icons were messed up, but additionally I couldn't set two different wallpaper, since the Desktop Preferences dialog only changed the wallpaper on monitor 1, but the image scaling took the whole Xserver screen as argument (3840x1200). There were two different dialogs popping up when using the right mouse click on the desktop. On monitor 2 the openbox popup, on monitor 1 the LXDE popup with limited options. No matter what I select, only the wallpaper for monitor 1 could be changed. My system setup is:

Operating System: Lubuntu 15.10
Graphics Adapter: Nvidia GeForce GTX 750
Driver:           Nvidia 352.63
Monitor 1:        HP LP2475w / connected via DVI-I-1 / 1920x1200
Monitor 2:        HP LP2475w / connected via DVI-D-0 / 1920x1200
XServer Screen:   3840x1200
PCManFM:          1.2.3

Anybody having the same problem or a solution how to fix this?