Context too fuzzy to make a bug report. Someone could help to track the root problem?

Rendering crashed just after creating empty .mp4 file.
Rendering succeeded when disabling video track containing title clips.
Viewing clip in project monitor is fine with all title clips.

My title clips contain only 1 text string on video track 2, and some video on video track 3.
Note: in my project, in project monitor, without any transition between title & video: the title appears on a black background, and no video is shown. I had to add a affine or add transition to see my video. On a new project, the video is shown in the title's background without any transition.
Question 1: what should be the "normal" behavior?

Error log extract of rendering crash:
(melt:2459): Gtk-WARNING **: Theme parsing error: gtk.css:597:14: not a number
(melt:2459): Gtk-WARNING **: Theme parsing error: gtk.css:597:14: Expected a string.
(melt:2459): Gtk-WARNING **: Theme parsing error: gtk.css:2286:18: Using Pango syntax for the font: style property is deprecated; please use CSS syntax

I was unable to reproduce the crash from a new project...

Other elements from my project context, exporting a very little project zone involving one .MP4 clip + one text title:
. DVD Wide screen project, 25 fps
. .dv miniDV Video Clips 25 fps and .MP4 Video Clips 720p 50 fps
. affine between Text clip and Video clip: crash / dissolve or addition: don't crash. Nothing: don't crash, but text on black background (no video).
. starting export at beginning of a cut, or one frame latter: crash. Starting export 2, 3, 4, 5, 6, 7, 8, 9 frame latter: don't crash.
. deleting first .dv clip starting video track from timeline: export don't crash, keeping this first .dv clip: export crash. Note: exporting zone doesn't contain any .dv clip, only one .MP4 clip.

The status: crash / don't crash is always very reproducible, but depends of a large set of conditions... and I cannot really find a logic inside! (Probably there is none).

Any idea to help me making other tests to identify root problem?

Kdenlive Version 16.12.3
MLT version 6.4.1, FFmpeg libraries
KDE Frameworks 5.31.0
Qt 5.7.1 (built against 5.7.1)
The xcb windowing system
UbuntuStudio 17.04 amd64.