OCI-59522
cannot open control file %0!s in disk group %1!s
Cause
The control file was already opened by another client in the cluster and the CLUSTER_DATABASE parameter was set to FALSE in either instance, or the old client resources were being cleaned.
Action
Ensure that the control file is not shared between two instances or, in case of Oracle Real Application Clusters, have all instances set the CLUSTER_DATABASE to TRUE. Once the configuration has been checked, and fixed if appropriate, retry the operation.