Strange issue after duplicating a database for standby usage

Hi,
I'm trying to duplicate a database for use in a dataguard (standby)-environment.
The duplicate succeeds but during the executing of the memory script

channel a4: restore complete, elapsed time: 01:41:12
Finished restore at 12-JUL-12

contents of Memory Script:
{
   switch clone datafile all;
}
executing Memory Script

RMAN-00571: ===========================================================
RMAN-00569: =============== ERROR MESSAGE STACK FOLLOWS ===============
RMAN-00571: ===========================================================

But no error message stack follows. Alertlog doesn't show any errors. Last message at this point is that it has fully restored the last datafile.
I've checked/doublechecked and restarted the duplicate a few times but without luck.

Anyone any clues?

greetings, Dick