By custom plugin you mean the Cinema4DBatch.py?
Just to jump in to discussion.
Maybe I missed some info on all these pages but here is the thing.
I’ve got identical problem and error described here.
As soon as I add any c4d plugin folder env var c4d stops working in deadline giving same problem.
Removing variable and using just standard c4d plugin folder everything works fine.
Weird thing is that error happens only on 2 computers while others works fine even with env var.
So for now sorted it out by removing env var from problematic computers and they are back rendering but still interesting to see what is going on.
Also to point out that render fails no mater what is used for rendering so it is not redshift thing.
Even putting all plugins from folder into one location and pointing var to that location it will crash as well.
c4d 19, deadline 10.0.9.4
Unfortunately, C4D Plugins Env can’t accept more than 1 path… I hope they will fix it in the next version
We decided to install RS locally on every machine because unchecking Use Batch, crashes the render nodes 90% of the times.
It’s confirmed that SP2 will fix the multiple paths problem. I’m on the Beta, I can try it. Will the submitter work with 19 SP2?
It had better! 
In all seriousness though, it should. If not, we’ll get it fixed as usual.