user11699: In the output of my video there is a short sequence where the whole video is solid red, with some corrupted text visible near the bottom. As usual in video editing software, this probably marks corrupted or missing frames. But why does it happen?
This happens in a location in the video where I put a Kdenlive Title Clip below a still image with a Fade Out effect. It does not happen with all possible combinations of rendering settings. In my case, it happens in a MP4 HD 1920×1080 project if setting it to be scaled to 1280×720 in the rendering settings and with output quality set to 26.
Kdenlive 19.12.3 under Ubuntu Linux 20.04 here.

user38352: Not sure if this could be the problem, but I got that exact rendering error from a similar scenario. I had title clips with fades as you describe, but they were on a video track that was above an empty gap in a lower video track. I just made a dummy clip to go beneath and fill that gap, and it cleared up the rendering error.
also, I had version 21.12.3 on Ubuntu 19.10 with the parallel processing on ( by default, didn't know it was an option) and encoder threads 0, also default. Since using a dummy clip was all it took to solve my issue, it means that the parallel processing bug has been fixed for 21.12.3 at least.

11699: <p>In the output of my video there is a short sequence where the whole video is solid red, with some corrupted text visible near the bottom. As usual in video editing software, this probably marks corrupted or missing frames. But why does it happen?</p>
<p>This happens in a location in the video where I put a Kdenlive Title Clip below a still image with a Fade Out effect. It does not happen with all possible combinations of rendering settings. In my case, it happens in a MP4 HD 1920×1080 project if setting it to be scaled to 1280×720 in the rendering settings and with output quality set to 26.</p>
<p>Kdenlive 19.12.3 under Ubuntu Linux 20.04 here.</p>
 38352: <p>Not sure if this could be the problem, but I got that exact rendering error from a similar scenario. I had title clips with fades as you describe, but they were on a video track that was above an empty gap in a lower video track. I just made a dummy clip to go beneath and fill that gap, and it cleared up the rendering error.</p>
<p>also, I had version 21.12.3 on Ubuntu 19.10 with the parallel processing on ( by default, didn't know it was an option) and encoder threads 0, also default. Since using a dummy clip was all it took to solve my issue, it means that the parallel processing bug has been fixed for 21.12.3 at least.</p>
