Problem with missing avconv

I'm running:
Pi V3 B+ Buster
OctoPrint version : 1.4.0
OctoPi version : 0.17.0
Octolapse v0.4.0
Pi Cam V2

As avconv does not exist in Buster I set path to FFMPEG = /usr/bin/ffmpeg and test is OK
But when I render I get the message: RenderError: return-code: Could not render movie, got return code -100:
The script at path '/usr/bin/avconv' could not be found

What is happening here? Octolapse seems to be trying to use avconv and not ffmpeg.