'Database is being updated by an instance which is not part of the current cluster' Error Message

Symptoms

An error message appears when accessing Confluence:

Background

Confluence comes with a cluster safety table (located in the database). This table exists even for non clustered environments. Every 30 seconds, Confluence checks this table and compares its value with the one it has in memory. If the new value differs from the one in memory, this error appears, and Confluence cannot proceed. This is the Cluster safety mechanism.

Causes

Though it appears to be a cluster related problem, this error occurs in non-clustered environments as well. There are three separate issues that all cause the same error message. Only one actually involves a clustered instance; the other two can happen to single node deployments:

1. Multiple instances of Confluence are deployed, connecting to the database.
2. A performance issue (usually invasive Garbage Collection) has suspended the cluster safety job from running. In Confluence 3.0.1 and 3.0.2, this is exacerbated and happens with a much higher frequency. See Cluster panics (Non Clustered Confluence 2.10.4, 3.0.1 and 3.0.2).
3. Communication between the nodes in a cluster has been severed.
4. In a single-node (non-clustered) deployment, there are two records in clustersafety table. 

Diagnosis and Resolutions

1. If the problem occurs shortly after startup in a single-node (non-clustered) deployment, see Cluster Panic due to Multiple Deployments.
2. If the problem occurs shortly after startup in a single-node (non-clustered) deployment and it wasn't causes by multiple deployments, see if clustersafety table (in the database) has more then one record. If so, just delete one of them.
3. If you are using 3.0.1 or 3.0.2, you are likely experiencing a bug. See Cluster panics (Non Clustered Confluence 2.10.4, 3.0.1 and 3.0.2).
4. If the problem happens spontaneously during production usage in a single-node (non-clustered) deployment, see Cluster Panic due to Performance Problems.
5. If the problem happens in a multi-node cluster, see Cluster Panic due to Multicast Traffic Communication Problem.