Abq warning: missing face identifier

[triacc]

Dear all,

I'm working on an assembly that consists of a couple of shell parts that are in contact with each other.
I keep getting the warning posted below, with the remark that I should not use this surface in contact, which unfortunately I have to. Does anyone know what this warning actually means and how I can get rid of it?

A face identifier has not been given for underlying element 1 instance sandwich_innen_shell-1. This surface should not be used in contact.

Thanks and greetings...