Data Guard :: V$DATAGUARD_STATUS - Log Transport Services Warning

OS read hat, oracle enterprise edition 11g release 2, dataguard 2 physical standby's

everything works fine but since 1 day we get following warnings from dataguard on the primary server:

SELECT * FROM V$DATAGUARD_STATUS;

FACILITY          SEVERITY     DEST_ID MESSAGE_NUM ERROR_CODE CAL TIMESTAM
------------------------ ------------- ---------- ----------- ---------- --- --------
MESSAGE
----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
Log Transport Services     Warning          0 309911     0 NO 01.11.12
RTC returned 2, disabling RTC for next 2 I/Os.

in the alert log it looks like this:

RTC returned 2, disabling RTC for next 2 I/Os.
Thu Nov 01 12:09:46 2012
RTC returned 2, disabling RTC for next 2 I/Os.
Re-enabling RTC following backoff interval 2.
Re-enabling RTC following backoff interval 2.
RTC returned 2, disabling RTC for next 2 I/Os.
RTC returned 2, disabling RTC for next 2 I/Os.
Re-enabling RTC following backoff interval 2.
Re-enabling RTC following backoff interval 2.
Thu Nov 01 12:11:16 2012
RTC returned 2, disabling RTC for next 2 I/Os.
Re-enabling RTC following backoff interval 2.