I doubt that LISTSERV itself is doing this.  The closest I can come to
this (with Eudora) is:

Subject:      [TEST] IMPRESOS PROYECTOS I+D DE INVESTIGACI=D3N B=C1SICA NO
              ORIENTADA
              DEL PLAN NACIONAL IDT  2000-2003 DISPONIBLES EN INTERNET ,

and if I have IETFHDR set instead of SUBJECTHDR (no attempt made by LISTSERV
to prettyprint the headers) I get

Subject: IMPRESOS PROYECTOS I+D DE INVESTIGACI=D3N B=C1SICA NO ORIENTADA DEL
         PLAN NACIONAL IDT  2000-2003 DISPONIBLES EN INTERNET ,

I suspect that the problem is in either or both the receiving or sending mail
client(s).  I will make the point that this subject line is extremely long
and some attempt should probably be made to shorten it anyway.

Nathan