Error Submitting TaskForm

Since some days we have problems to submit taskforms. This error is critical for our production environment. We can't reproduce it in our test environment.

The log shows that the update of IdentityLink Table fails:

1. 1. 1. Error updating database. Cause: org.apache.ibatis.type.TypeException: Error setting null for parameter #1 with JdbcType OTHER . Try setting a different JdbcType for this parameter or a different jdbcTypeForNull configuration property. Cause: java.sql.SQLException: Ungültiger Spaltentyp: 1111
      2. The error may involve org.camunda.bpm.engine.impl.persistence.entity.IdentityLinkEntity.deleteIdentityLink-Inline
      3. The error occurred while setting parameters
      4. SQL: delete from ACT_RU_IDENTITYLINK where ID_ = ?
      5. Cause: org.apache.ibatis.type.TypeException: Error setting null for parameter #1 with JdbcType OTHER . Try setting a different JdbcType for this parameter or a different jdbcTypeForNull configuration property. Cause: java.sql.SQLException: Ungültiger Spaltentyp: 1111'. Flush summary:
         [
         INSERT HistoricVariableInstanceEntity[26696794]
         INSERT HistoricVariableInstanceEntity[26696796]
         INSERT HistoricVariableInstanceEntity[26696798]
         INSERT HistoricVariableInstanceEntity[26696805]
         INSERT HistoricVariableUpdateEventEntity[26696795]
         INSERT HistoricVariableUpdateEventEntity[26696797]
         INSERT HistoricVariableUpdateEventEntity[26696799]
         INSERT HistoricVariableUpdateEventEntity[26696806]
         INSERT HistoricTaskInstanceEventEntity[26696809]
         INSERT HistoricIdentityLinkLogEventEntity[26696811]
         INSERT HistoricFormPropertyEventEntity[26696800]
         INSERT HistoricFormPropertyEventEntity[26696801]
         INSERT HistoricFormPropertyEventEntity[26696802]
         INSERT HistoricActivityInstanceEventEntity[ExclusiveGateway_1yqzp67:26696807]
         INSERT HistoricActivityInstanceEventEntity[partner_gefunden_gateway:26696803]
         INSERT HistoricActivityInstanceEventEntity[partner_neu_anlegen:26696808]
         INSERT HistoricActivityInstanceEventEntity[pruefe_partner_neu_in_bearbeitung:26696804]
         INSERT TaskEntity[26696809]
         INSERT VariableInstanceEntity[26696794]
         INSERT VariableInstanceEntity[26696796]
         INSERT VariableInstanceEntity[26696798]
         INSERT VariableInstanceEntity[26696805]
         INSERT IdentityLinkEntity[26696810]
         DELETE EventSubscriptionEntity[26696570]
         DELETE IdentityLinkEntity[null]
         DELETE TaskEntity[26696572]
         UPDATE ExecutionEntity[26696495]
         DELETE ExecutionEntity[26696569]
         UPDATE HistoricActivityInstanceEventEntity[partner_suchen:26696571]
         UPDATE HistoricTaskInstanceEventEntity[26696572]
         ]