This happens when placing svg images in the timeline, then when exporting the video to any format, the image does not appear at all only text saying in capital leteres INVALID... (it does not happen if the svg image is placed in a title clip and this clip placed in the timeline) but inserting the svg directly in the timeline would be great for some compositing basic effects ...
Please try with the actual Kdenlive version 18.08.2
It seems that this report is related to very old unmaintained version. A lot changed since then, especially the timeline got a complete rewrite and it is likely that this has been fixed. Feel free to reopen this bug or create a new report if this is still happening with the latest version (https://kdenlive.org/en/download/)