Apparently, there is a paper
M. Sablik, Final part of the answer to a Hilbert's question. Functional Equations - Results and Advances. Edited by Z. Daróczy and Zs. Páles, Kluwer Academic Publishers 2002, 231-242.
however I struggle to understand what does it have to do with Lie groups and Hilbert's fifth problem. Is it something deeper I fail to see here?