Not entirely sure if this is a new issue, but after collecting the essence from a CyberZombie from cyberware (which registers the cyberzombie as a zombie type)

the spawner says nullRF, is there a config I need to change or has it just not worked?

i have having the same problem with all mobs as well

The same thing. I added couple of mobs to config, but it says "nullRF" regardless of that all needed strings are present.

It looks like the more strings in config the worse RFTools is working.

Can you post your rftools.cfg?

This issue has been marked as needing a response from the reporter for over 2 weeks without receiving one. As such, it has been automatically closed.

For anyone still looking for a solution to this problem, in my case, it was caused by a problem with the configs. My pack has existed since MC 1.9.4 and has been through many changes, so the config was out of sync. I fixed it by deleting the config, allowing it to be regenerated, and then working with the newly generated config file.

I am having the same problem with all mobs, including vanilla mobs: Enderman, Zombie, EnderMini (from Ender Zoo), etc. When the syringe is inserted, it shows that it requires nullRF and shows nothing where the required matter should appear.
RFTools 6.10
Forge 13.20.0.2315
MC 1.11.2
Pack https://minecraft.curseforge.com/projects/danny-sirenuse-pack

Perhaps this is related to #1199