Thanks a lot for your help, this solution works for me! :-)
Damn we were already looking for more exotic solutions,
I'm quite happy that we can solve it this way.
Indeed if I run the query distributed with openquery,
and I put To_NChar() around the column that gave the error
it works smoothly. (below the working query)
FROM OPENQUERY(SERV_VISION, 'SELECT LANG_DIV
,To_NChar(CAUSE_NM) as CAUSE_NM
,CLASS_CD2 from VISION.INV_M008')
Thanks a lot!