You may have hit bug# 3597439, check it out in Metalink.

Here is an excerpt:

ALTER DATABASE RECOVER MANAGED STANDBY DATABASE command will fail with
ORA-1153, "incompatible media recovery active" if an RMAN backup is
in progress.

Workaround:
Start managed recovery before starting RMAN backup,
(They will run at the same time, but only if managed recovery is started first)
or
Wait until the RMAN backup is complete before starting managed recovery.