How to solve copy / paste problem of Keepass2 in Ubuntu

Linux has two ways of copying and pasting:

1. select text using the mouse to copy it, then middle-click (or left and right click together) with the mouse to paste it
2. select text, then press Ctrl-C to copy it, then press Ctrl-V to paste it

You can use an intermediary which can use both clipboards, for example, copying from Keepass, pasting into gEdit, copying from gEdit, then pasting into the place you want.

Or use autocutsel

$ sudo apt-get install autocutsel
$ autocutsel &
$ autocutsel -s PRIMARY &