There is a bug in 8.1.6 which produces this problem (1226437). The workaround is to recreate the primary and standby controlfiles and then apply the logs.

Take a look at these metalink threads:

http://support.oracle.co.uk/metalink..._id=418186.999

http://support.oracle.co.uk/metalink...G&p_id=1226437

HTH