I've tried everything, I've reset the configuration, updated protocollib, switched to paper, put only wildstacker and nothing has worked :(

happens with magma cube -> https://youtu.be/bN_dXNW4uKA

error build 25 https://mclo.gs/qgUneZi

The error comes from BLOCK_DISPLAY, which doesn't make a lot of sense.
I suspect it's another plugin somehow messing with slimes.

Try it on a server with only WildStacker installed, default config files.
Let me know if the error still occurs.

I've already deleted my configuration and made everything from scratch, then when I restart the server the bug happens, could you see my configuration and see what I did wrong it doesn't make sense I haven't touched anything just update the jar, it's been about 5-6 months now

i've been using this configuration for 5-6 months, and this has never happened, the only thing i added was the enchanted_book in the items, that's it, i did the test only with wildstacker and when i put my configuration the bug happens.

https://prnt.sc/IB2FMN_NQnme

another thing, I deleted the old configuration, started a new one, put all my settings in the new one and restarted the server and still the bug happened, it doesn't make sense

well I did all the tests there are, and the solution was to activate the option:

Settings related to overriding spawners.

spawners-override:
# Should spawners behavior be overridden by WildStacker?
# When enabled, you should see better performance from spawners.
enabled: true
# Configure spawn conditions for entities that are spawned by spawners
# You can find more information regarding this by visiting the official wiki:
# https://wiki.bg-software.com/#/wildstacker/spawn-conditions/
spawn-conditions:
EXAMPLE_MOB: []

  solved the problem, but what happens now is that the slime doesn't join up with the small slime, it stays separate, generating large, medium and small slimes.
  
  video: https://youtu.be/VPEJNKl5zXo
  
  solved by disabling the slime-size: false option
  
  I took a new configuration and put mine in, and I activated this option and it worked, but if I deactivate it the bug happens, this is proof that the problem is not with me, it doesn't make any sense.
  
  
  there's just one problem with this, there's still the same bug but the other way around, now it happens when I pull 1 big slime next to a small one, and it needs to be spawn reason: SPAWNER
  
  small to big no bug:  https://www.youtube.com/watch?v=3pLrowXUPnw ( it's perfect )
  
  now look at the opposite 
  
  https://youtu.be/NGgQBvRD6c4

Check out latest dev build, let me know if it works even if you disable spawners-override.

yes it's been fixed, this small and large slime thing shouldn't be optional to choose, it should be by default that the small slime never enters the large one, this is the second time this has happened, after a few updates

Hopefully it will not happen again :)