You can add this line at the begining of your source code
SESSIONEBUG-ALERT = TRUE.
And press help to know where this issue is fired.
The line number come from compile [PGM] debug-list [debug-list file].
You can avoid it by testing the length of the LONGCHAR before trying to convert it with STRING() it or by using SUBSTRING() to take the first X bytes. A MESSAGE with more than a few hundred bytes is probably useless anyway so I often code something like: