I gave kdenlive a test here.
One thing I noticed is it is not installing the oxygen-icon-theme and it should.
However, I was able to add clips in and then put them on the time line without issue.
Sometimes I'd get an info popup letting me know the clip did not match the size of the profile but it still allowed me to load it and use it in the video.
I'll let hata_ph know about the oxygen-icon issue.
What video formats are you trying to add in and what Video Profile did you presect at start.