Pulling my hair out, cannot get any encodes to start.

[frencoder]

Okay.
According to the debug ouput processing the Vapoursynth script fails.
And since I only look at the debug outputs of the latest release, that's as far as I go there.

My advice would be to:
a. uninstall the old Hybrid Verison and all it's files
b. since you are on a Debian based system, use the latest Hybrid Linux64bit package (with base tools and most dependencies).
c. use https://github.com/Selur/hybrid-vapoursynth-addon to setup Vapoursynth
d. before starting an encoding, check that the Vapoursynth Preview works
If the Vapoursynth Preview fails, it should show  some error which might help to figure out what is going wrong.
If you use the latest version and the Vapoursynth Preview does work, try encoding again and in case it fails, create a debug output level 9 of the job processing.

My guess is that the problem is the Vapoursynth setup and the usage of 'vsfilters_env.sh' which was removed in newer versions, which is why you should make sure to remove Hybrid and Vapoursynth completely.

Cu Selur

Still no dice. 
https://pastebin.com/YxwgbtZA
Tells me xxx.stats was not created. I do see an error in the log 

y4m [error] bad sequence header magic