diff --git a/knora-ontologies/knora-base.ttl b/knora-ontologies/knora-base.ttl
index 549d748078e..0318fd49373 100644
--- a/knora-ontologies/knora-base.ttl
+++ b/knora-ontologies/knora-base.ttl
@@ -19,7 +19,7 @@
     rdf:type           owl:Ontology ;
     rdfs:label         "The Knora base ontology"@en ;
     :attachedToProject knora-admin:SystemProject ;
-    :ontologyVersion   "knora-base v25" .
+    :ontologyVersion   "knora-base v26" .
 
 
 #################################################################
@@ -1820,7 +1820,7 @@
                       owl:cardinality "1"^^xsd:nonNegativeInteger ],
                     [ rdf:type           owl:Restriction ;
                       owl:onProperty     :lastModificationDate ;
-                      owl:maxCardinality "1"^^xsd:nonNegativeInteger ],
+                      owl:cardinality "1"^^xsd:nonNegativeInteger ],
                     [ rdf:type           owl:Restriction ;
                       owl:onProperty     :deleteDate ;
                       owl:maxCardinality "1"^^xsd:nonNegativeInteger ],
diff --git a/webapi/src/main/scala/org/knora/webapi/package.scala b/webapi/src/main/scala/org/knora/webapi/package.scala
index 6c35258615f..ab8c8b53535 100644
--- a/webapi/src/main/scala/org/knora/webapi/package.scala
+++ b/webapi/src/main/scala/org/knora/webapi/package.scala
@@ -11,7 +11,7 @@ package object webapi {
    * The version of `knora-base` and of the other built-in ontologies that this version of Knora requires.
    * Must be the same as the object of `knora-base:ontologyVersion` in the `knora-base` ontology being used.
    */
-  val KnoraBaseVersion: String = "knora-base v25"
+  val KnoraBaseVersion: String = "knora-base v26"
 
   /**
    * `IRI` is a synonym for `String`, used to improve code readability.
diff --git a/webapi/src/main/scala/org/knora/webapi/store/triplestore/upgrade/RepositoryUpdatePlan.scala b/webapi/src/main/scala/org/knora/webapi/store/triplestore/upgrade/RepositoryUpdatePlan.scala
index ebcb51635e5..0a6ed383921 100644
--- a/webapi/src/main/scala/org/knora/webapi/store/triplestore/upgrade/RepositoryUpdatePlan.scala
+++ b/webapi/src/main/scala/org/knora/webapi/store/triplestore/upgrade/RepositoryUpdatePlan.scala
@@ -1,7 +1,6 @@
 package org.knora.webapi.store.triplestore.upgrade
 
 import com.typesafe.scalalogging.Logger
-
 import org.knora.webapi.store.triplestore.upgrade.plugins._
 
 /**
@@ -53,7 +52,8 @@ object RepositoryUpdatePlan {
       PluginForKnoraBaseVersion(versionNumber = 22, plugin = new UpgradePluginPR2081(log)),
       PluginForKnoraBaseVersion(versionNumber = 23, plugin = new UpgradePluginPR2094(log)),
       PluginForKnoraBaseVersion(versionNumber = 24, plugin = new NoopPlugin), // PR 2076
-      PluginForKnoraBaseVersion(versionNumber = 25, plugin = new UpgradePluginPR2255(log))
+      PluginForKnoraBaseVersion(versionNumber = 25, plugin = new UpgradePluginPR2255(log)),
+      PluginForKnoraBaseVersion(versionNumber = 26, plugin = new UpgradePluginPR2288(log))
       // KEEP IT ON THE BOTTOM
       // From "versionNumber = 6" don't use prBasedVersionString!
     )
diff --git a/webapi/src/main/scala/org/knora/webapi/store/triplestore/upgrade/plugins/UpgradePluginPR2288.scala b/webapi/src/main/scala/org/knora/webapi/store/triplestore/upgrade/plugins/UpgradePluginPR2288.scala
new file mode 100644
index 00000000000..3b54cb1eb21
--- /dev/null
+++ b/webapi/src/main/scala/org/knora/webapi/store/triplestore/upgrade/plugins/UpgradePluginPR2288.scala
@@ -0,0 +1,34 @@
+package org.knora.webapi.store.triplestore.upgrade.plugins
+
+import com.typesafe.scalalogging.Logger
+import org.knora.webapi.messages.util.rdf.{RdfFeatureFactory, RdfModel, Statement}
+import org.knora.webapi.store.triplestore.upgrade.UpgradePlugin
+
+import scala.concurrent.duration.DurationLong
+
+class UpgradePluginPR2288(log: Logger) extends UpgradePlugin {
+  private val nodeFactory = RdfFeatureFactory.getRdfNodeFactory()
+
+  private val creationDateIri = nodeFactory.makeIriNode("http://www.knora.org/ontology/knora-base#creationDate")
+  private val lastModDateIri  = nodeFactory.makeIriNode("http://www.knora.org/ontology/knora-base#lastModificationDate")
+
+  override def transform(model: RdfModel): Unit = {
+    val t0 = System.nanoTime()
+    log.info(s"Starting ${this.getClass.getSimpleName}")
+
+    val statementsWithCreationDateIri =
+      LazyList.from(model.find(subj = None, pred = Some(creationDateIri), obj = None))
+
+    val subjectHasNoLastModificationDate: Statement => Boolean = statement =>
+      model.find(subj = Some(statement.subj), pred = Some(lastModDateIri), obj = None).isEmpty
+
+    val newStatements = statementsWithCreationDateIri
+      .filter(subjectHasNoLastModificationDate)
+      .map(s => nodeFactory.makeStatement(s.subj, lastModDateIri, s.obj, s.context))
+
+    model.addStatements(newStatements.toSet)
+
+    log.info(s"Created ${newStatements.size} new statements:\n${newStatements.mkString("++ ", "\n", "")}")
+    log.info(s"Finished ${this.getClass.getSimpleName} in " + (System.nanoTime() - t0).nanos.toCoarsest.toString())
+  }
+}