[HFSS] What does N/A mean for delta S?

'Not applicable' =usual meaning.

For the first pass at that frequency, there is no previous S-parameter value to compare with, so deltaS cannot be calculate -> not applicable.

I understand the first pass. But why the 8th pass? What possible reason could there be for delta S to be not calculated

Additional meshing frequency oder additional convergence criteria?

No. Only the simple delta S criteria, only a single frequency adaptive meshing.

I have difficulties to believe that - look at deltaS which jumps back to very large values after the N/A in pass 8.
I'm not an HFSS user, but I only see that behaviour in other FEM if there is multiple criteria or frequencies for mesh refinement.