It could be lock issue, rename below files
cd $EBS_DOMAIN_HOME/servers/oacore_server1/data/store/diagnostics
WLS_DIAGNOSTICS000000.DAT
mv WLS_DIAGNOSTICS000000.DAT WLS_DIAGNOSTICS000000.DAT_old
You can now restart the managed server
It could be lock issue, rename below files
cd $EBS_DOMAIN_HOME/servers/oacore_server1/data/store/diagnostics
WLS_DIAGNOSTICS000000.DAT
mv WLS_DIAGNOSTICS000000.DAT WLS_DIAGNOSTICS000000.DAT_old
You can now restart the managed server