Thanks to thekeyboardbum for finding out that it is a configure problem of mlt.
After a lot of testing, I found out that it is the kde USEFLAG, which causes the problems. When it is not set, the configure parameter "--disable-kdenlive" is set and then openshot crashes. So, for some strange reason, mlt has to be build with kdenlive enabled in order to work correctly with openshot.
Thanks to thekeyboardbum for finding out that it is a configure problem of mlt.
After a lot of testing, I found out that it is the kde USEFLAG, which causes the problems. When it is not set, the configure parameter "--disable- kdenlive" is set and then openshot crashes. So, for some strange reason, mlt has to be build with kdenlive enabled in order to work correctly with openshot.