Strange error in trace file for NSAn standby process
At first glance, in MOS there's a related known bug with that message in 11.2.0.1 on Solaris (Bug 10331312)  and 11.2.0.2 on Linux x86-64 (Bug 14016842 , this one refilled for review) both referenced by Doc Id 1308804.1.

However it doesn't provides any source of cause. As workaround they recommend to disable the log_archive_trace parameter to = 0 (default), as those messages can be ignored.