After upgrading my virtual dev-station from Debian 8 "Jessie"to Debian 9 "Stretch" (using XFCE desktop) I could not make connections in the PLC editor by using the middle mouse button, though the button worked when clicking on the desktop. Also I experienced a way longer desktop initializing phase, but never thought there might be a relation between those issues.
The solution was to uninstall the packet xserver-xorg-input-libinput and only have the packet xserver-xorg-input-synaptics installed. Tried vice-versa without success.
By dropping xserver-xorg-input-libinput the desktop environment loads way quicker, too.