I use lubuntu 17.4 and the newest kdenlive (16.12.3).
Whenever I open a project in lxde (standard desktop for lubuntu), the "file open dialog" opens, but hangs ... forever. I have to close the dialog window to edit my projects.

I can edit videos, no problem with that. Even I can "open recent documents"; but "open file" does not work, so I can not open older projects.

Today I installed "plasma"-Desktop and see: using "plasma" everything works perfect.
When I login using lxde again, I still have the same problem; so it cannot be an issue of a missing library or a missing package. It has to do with the lxde under lubuntu. Any Idea, what I have to change in lxde?

For me, I have now a workaround: Use plasma for kdenlive and lxde for all other activities, because I am used to lxde

Any idea?