Problems with Prototyping and Calling RPGIII Programs

Hi,

I had some strange problem calling RPGIII programs through CALLP and
Prototypes.
Until today our application consists of about 50% of RPGIII Programs.
All our RPGIII programs only have 1 parameter (a externally described data
structure that is embedded as copy member in the I-specs).
The call to these programs looks as follows:

To call these programs from RPGIV free format we created a prototype that
looks as follows:
 /If Not Defined(DSIN)
 /DEFINE DSIN
D DSIN          E DS                  EXTNAME(LLDSIN)  
 /ENDIF

D MyPGM           S                   like(RefPGM) 

D PgmDSIN         PR                  ExtPGM(MyPGM)     
D   ParmDSIN                          like(DSIN)     
                                                   
Calling RPGIII program with CallP causes a failure with MCH3601. 
(Even after recompiling the RPGIII programs and the calling RPGIV, 
and restarting the session. Library list is ok and there is only 1 version
of the program)
Calling RPGIV with the same prototype causes no problems.

The call looks as follows:
/Free
  MyPGM = 'LL4711R';
  Monitor;
     PgmDSIN(DSIN);
  On-Error;
  EndMon;
/End-Free

If I change the call to an CALL in non Free-Format, 
the RPGIII program is executed without problems.

It seems that some modifications and recompiles of the calling RPGIV
procedures cause this problem.
Originally these calls worked.

Has somebody had comparable problems?
What can we do (with the exception of converting the RPGIII programs) to
prevent this failures?

Birgitta