i think it's a wrong slash direction ..
instead of :
/script GuildRosterSetPublicNote(3,"124cffffff11".."BLABLA")
you should write :
/script GuildRosterSetPublicNote(3,"\124cffcc0000".."BLABLA")
Look for the slash AFTER 3,"
Tested on WoW 2.2.3, Private server 2.2.3 (Ascent)