There are multiple causes for this issue. Start with the parent article Confluence will not start due to fatal error in Confluence cluster.
Confluence throws the following error on a clustered deployment:
Fatal error in Confluence cluster: Database is being updated by an instance which is not part of the current cluster. You should check network connections between cluster nodes, especially multicast traffic.
Nodes in the cluster cannot communicate. This may be due to a failed switch or other network failure.
For Confluence 5.4 and earlier see Cluster Troubleshooting.
For Confluence Data Center 5.6 and later see Recovering from a Data Center cluster split-brain