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Well I think you can close this topic as the answer is that makeindex does not allow \! and needs \negthinspace instead.
Thanks!

Well but \! is still a command, or not? So it should recognize it imo and only interpret a single ! as a sub-index entry.

I found a workaround which is using \negthinspace instead of \!. So it seems to be a bug within nomencl. I will give an example asap.

Hello, I have the following symbol:
${^B\!}f_i$ which describes a vector in the frame {B}.
I declare the symbol to nomencl with
\nomenclature[yx ]{${^B\!}f_i$}{Vector $f$ in \{B\} frame} .

The problem is that nomencl seems to misinterpret the "\!" (which reduces the space between the index and ...