Can't add transparent video (lower third) in Kdenlive

Hello,

I created a lower third video with Viddyoze

How it's supposed to look:
https://s3-us-west-2.amazonaws.com/v...eo_658x370.mp4

Example of an animated lower third:
https://gofile.io/d/Esmetz
or
https://www.dropbox.com/s/opl3e7yfcs...Third.mov?dl=0

I want to add the transparent lower third to my own videos but I can't get it done. It's never transparent. I have already tried many effects and many transitions, like Cairo Affine Blend and many blend modes etc. But it's just never transparent. Usually the whole lower third video has a black background that covers the video behind. Or I can't see the lower third at all.

Usually I use Premiere Pro and there it's just drag and drop and done. However, I would prefer to do my videos on Linux. I also tried it on Olive on Linux and it works out of the box, without any plug-ins or effects. But Olive is still in beta and has many other bugs.

How can I add the lower third video to my own video?

Thank you!

Kdelive 20.04.1 on Manjaro