It is impossible to store any data in the Cassandra cache store, because it is created as read-only, even when specifying ignoreModifications = false. See the attached test case:

[INFO ] ISPN000078: Starting JGroups Channel (org.infinispan.remoting.transport.jgroups.JGroupsTransport)
[INFO ] ISPN000088: Unable to use any JGroups configuration mechanisms provided in properties {}. Using default JGroups configuration! (org.infinispan.remoting.transport.jgroups.JGroupsTransport)
[INFO ] ISPN000094: Received new cluster view: [GNELA182-53637|0] [GNELA182-53637] (org.infinispan.remoting.transport.jgroups.JGroupsTransport)
[INFO ] ISPN000079: Cache local address is GNELA182-53637, physical addresses are [fe80:0:0:0:64f5:4fad:cd2e:b5cf:51350] (org.infinispan.remoting.transport.jgroups.JGroupsTransport)
[INFO ] ISPN000128: Infinispan version: Infinispan 'Delirium' 5.2.6.Final (org.infinispan.factories.GlobalComponentRegistry)
[DEBUG] automatically create keyspace (org.infinispan.loaders.cassandra.CassandraCacheStore)
[INFO ] ConnectionPool initialized. (net.dataforte.cassandra.pool.ConnectionPool)
[DEBUG] cleaning up expired entries... (org.infinispan.loaders.cassandra.CassandraCacheStore)
[TRACE] purgeInternal (org.infinispan.loaders.cassandra.CassandraCacheStore)
[INFO ] ConnectionPool initialized. (net.dataforte.cassandra.pool.ConnectionPool)
[DEBUG] started (org.infinispan.loaders.cassandra.CassandraCacheStore)
[TRACE] Ignoring clear invocation (org.infinispan.loaders.decorators.ReadOnlyStore)
[INFO ] ISPN000031: MBeans were successfully registered to the platform MBean server. (org.infinispan.jmx.CacheJmxRegistration)
[INFO ] creating test1 (com.agfa.aca.PassivationTest)
[DEBUG] Key 'entry_test1' not found (org.infinispan.loaders.cassandra.CassandraCacheStore)
[TRACE] Ignoring removal of key (org.infinispan.loaders.decorators.ReadOnlyStore)
[INFO ] created test1 (com.agfa.aca.PassivationTest)
[INFO ] creating test2 (com.agfa.aca.PassivationTest)
[DEBUG] Key 'entry_test2' not found (org.infinispan.loaders.cassandra.CassandraCacheStore)
[INFO ] passivating test1 (com.agfa.aca.PassivationTest)
[TRACE] Ignoring store invocation (org.infinispan.loaders.decorators.ReadOnlyStore)
[INFO ] passivated test1 (com.agfa.aca.PassivationTest)
[TRACE] Ignoring removal of key (org.infinispan.loaders.decorators.ReadOnlyStore)
[INFO ] created test2 (com.agfa.aca.PassivationTest)
[DEBUG] Key 'entry_test1' not found (org.infinispan.loaders.cassandra.CassandraCacheStore)

I tracked this bug down to the create method in CassandraCacheStoreConfigurationBuilder

   @Override
   public CassandraCacheStoreConfiguration create() {
      List<CassandraServerConfiguration> remoteServers = new ArrayList<CassandraServerConfiguration>();
      for (CassandraServerConfigurationBuilder server : servers) {
         remoteServers.add(server.create());
      }
      return new CassandraCacheStoreConfiguration(autoCreateKeyspace, configurationPropertiesFile, entryColumnFamily,
            expirationColumnFamily, framed, remoteServers, keyMapper, keySpace, password, sharedKeyspace, username,
            readConsistencyLevel, writeConsistencyLevel, autoCreateKeyspace, sharedKeyspace, purgerThreads, framed,
            autoCreateKeyspace, TypedProperties.toTypedProperties(properties), async.create(), singletonStore.create());
   }

Note how autoCreateKeyspace is used in stead of purgeInternal and ignoreModifications, so if you explicitly set autoCreateKeyspace = false you can work around the issue.
This is tested on 5.2.6, all versions from 5.2 on have the same error.