This message about a negative surrogate is only a warning and will not be a problem. Negative surrogates are not a problem unless you begin to see a large number of them and especially if you see the number of negative surrogates increasing at a fast pace. We recommend to disable extracting the negative surrogates to local model by following Plex.ini file setting:
Mask Negative Surrogates=1
The negative surrogate warning will be there as negative surrogates are present in group model; extract process will always log the warning.
If negative triple surrogates are detected within the group model on an extract, the error appears. By default this is displayed as a dialog and in the message log.
The dialog can be switched off with the following Plex.ini file setting:
Show Negative Surrogates Error Dialog=0
Show High Surrogate Dialog=0