When use a friendly key on a chest the friendly mimic is generated, but it doesn't move neither you can interact with him.

I've noticed this as well. Restarting the server seems to temporarily fix this but then it eventually gets frozen again.

This is what the Probably Chests dev said when this pops up. :

This is an issue with geckolib and iris that I cannot fix caused by things like the entity shadows feature in shaders. I found that disabling entity shadows in the shader config fixes it.

Have you tried that yet?