Was this problem either sorted out ? I have a similiar problem:
RMAN gives:
ORA-19502: write error on file "GD1_DBF_b3i79lck", blockno 1 (blocksize=512)
ORA-27030: skgfwrt: sbtwrite2 returned error
ORA-19511: Error received from media manager layer, error text:
ANS1017E (RC-50) Session rejected: TCP/IP connection failure

and tdpio.log gives:
SBT-89758 12/09/2006 09:27:22 send2.cpp(544): sbtbackup(): Exit - tdpoSession() failed. dsmHandle = 0, rc = -50
SBT-89364 12/11/2006 18:23:42 send2.cpp(383): sbtwrite2(): Exit - DSMSENDDATA() failed. dsmHandle = 1

any ideas ?