I've been happily using Kdenlive 19.12.3 AppImage on Ubuntu64 18.0.4 for a couple
of weeks, rendering .mp4 files with no problem. Suddenly Render crashes on starting with
"Rendering of /Agrar2.mp4 crashed
No LADSPA plugins were found! Check your LADSPA_PATH environment variable.
[consumer avformat] Could not open '/Agrar2.mp4' "

What happened? Help!

THIS IS NOT A REPLY Here are some more details.

I looked closely at the error message, and generated the .mlt script.
This is how it starts:

<mlt title="Anonymous Submission" LC_NUMERIC="en_US.UTF-8" producer="main_bin" version="6.21.0" root="/home/peter/Videos/Vortrag/Agrar2">
<profile frame_rate_num="25" sample_aspect_num="1" display_aspect_den="9" colorspace="709" progressive="1" description="HD 1080p 25 fps" display_aspect_num="16" frame_rate_den="1" width="1920" height="1080" sample_aspect_den="1"/>
<consumer f="mp4" g="15" crf="23" progressive="1" target="/Agrar2.mp4" threads="0" real_time="-1" mlt_service="avformat" vcodec="libx264" ab="192k" movflags="+faststart" bf="2" preset="faster" acodec="aac" in="0" out="3924"/>

Render seems to crash here, at <consumer..."avformat". Why?
Do I have the Mlt plug-in? I suppose I do, since Render worked
until yesterday. Do I have the LADSPA plug-in? I find no trace
in /usr/lib, but what do I know? As you can see, I'm in way over
my head, and I'd appreciate some help.