I have resolved this issue by removing these lines from my python code:
#set pyvirtual display
display = Display(visible=0, size=(800, 600))
display.start()
and adding the following lines:
import os
os.environ['DISPLAY'] = ':1'
This way the python script uses the already open Xvfb process setup on display 1 instead of starting a new Xvfb server process.