After some trials I found to my surprise that, irrespective of iconv, the same error occurred with both entirely new txt iso-8859-7 files and old ARGFILES in the same encoding from about a year ago which worked perfectly.
Preliminary findings point to an important change in exiftool in ver. 6.70 about the treatment of encoded characters. I am still trying to understand it's implications. From a first reading it seems to cater either for utf-8 or cp1252. What about cp1253 (iso-8859-7)?