Your raw2 cell array contains numerical values in column 1. strcat interpret these values as character codes (e.g. it translates 65 into A) and is warning you that some of them are not valid character codes (because they're not integers or not in the valid range of integers).
Most likely, you didn't want to perform strcat on that column of the cell array. What are you trying to do?
Also suspicious is your passing of the 'rows' to strcat, wasn't this meant as a second argument to unique instead?
That is did you mean?
[~, id_raw2] = unique(raw2(:, 1), 'rows');