This happens if you are running a kdenlive script from the shell of a remote machine without using SSH "-X" forwarding and it will also cause any areas where you write text to be a white screen for that duration.
melt FusionFestival.kdenlive
No LADSPA plugins were found!
Check your LADSPA_PATH environment variable.
[producer_xml] failed to load transition "qtblend"
[producer_xml] failed to load transition "qtblend"
[producer_xml] failed to load transition "qtblend"
+-----+ +-----+ +-----+ +-----+ +-----+ +-----+ +-----+ +-----+ +-----+
|1=-10| |2= -5| |3= -2| |4= -1| |5= 0| |6= 1| |7= 2| |8= 5| |9= 10|
+-----+ +-----+ +-----+ +-----+ +-----+ +-----+ +-----+ +-----+ +-----+
+---------------------------------------------------------------------+
| H = back 1 minute, L = forward 1 minute |
| h = previous frame, l = next frame |
| g = start of clip, j = next clip, k = previous clip |
| 0 = restart, q = quit, space = play |
+---------------------------------------------------------------------+
WARNING: QApplication was not created in the main() thread.
Current Position: 967
So if executing a script remotely you have to ssh in like this:
ssh -X user@host.com
kdenlive, ladspa, plugins, ladspa_path, variable, producer_xml, transition, qtblend, shell, ssh, quot, forwarding, areas, text, duration, melt, fusionfestival, previous, clip, restart, qapplication, executing, remotely, user,