diff --git a/Makefile b/Makefile
index 3613c1044..fe11a4384 100644
--- a/Makefile
+++ b/Makefile
@@ -73,6 +73,7 @@ $(GENERATED_CRDS): $(GENERATED)
 	@sed -i -e 's/listKind: PostgresqlList/listKind: postgresqlList/' manifests/postgresql.crd.yaml
 	@hack/adjust_postgresql_crd.sh
 	@mv manifests/acid.zalan.do_operatorconfigurations.yaml manifests/operatorconfiguration.crd.yaml
+	@hack/adjust_operatorconfiguration_crd.sh
 	@mv manifests/acid.zalan.do_postgresteams.yaml manifests/postgresteam.crd.yaml
 	@cp manifests/postgresql.crd.yaml pkg/apis/acid.zalan.do/v1/postgresql.crd.yaml
 	@cp manifests/operatorconfiguration.crd.yaml pkg/apis/acid.zalan.do/v1/operatorconfiguration.crd.yaml
diff --git a/charts/postgres-operator/crds/operatorconfigurations.yaml b/charts/postgres-operator/crds/operatorconfigurations.yaml
index 790b9effc..423fd4a89 100644
--- a/charts/postgres-operator/crds/operatorconfigurations.yaml
+++ b/charts/postgres-operator/crds/operatorconfigurations.yaml
@@ -709,6 +709,9 @@ spec:
                     type: integer
                 type: object
               maintenance_windows:
+                items:
+                  pattern: '^\ *((Mon|Tue|Wed|Thu|Fri|Sat|Sun):(2[0-3]|[01]?\d):([0-5]?\d)|(2[0-3]|[01]?\d):([0-5]?\d))-((2[0-3]|[01]?\d):([0-5]?\d)|(2[0-3]|[01]?\d):([0-5]?\d))\ *$'
+                  type: string
                 type: array
               major_version_upgrade:
                 description: MajorVersionUpgradeConfiguration defines how to execute
diff --git a/hack/adjust_operatorconfiguration_crd.sh b/hack/adjust_operatorconfiguration_crd.sh
new file mode 100755
index 000000000..61fb4ac03
--- /dev/null
+++ b/hack/adjust_operatorconfiguration_crd.sh
@@ -0,0 +1,22 @@
+#!/usr/bin/env bash
+
+# Hack to adjust the generated operatorconfiguration CRD YAML file and add
+# missing field settings which can not be expressed via kubebuilder markers.
+#
+# Injections:
+#
+#  * type: string and pattern for the maintenance_windows items.
+#    The MaintenanceWindow type marshals to/from a string (see marshal.go) but
+#    the field is declared `+kubebuilder:validation:Schemaless` /
+#    `+kubebuilder:validation:Type=array`, so controller-gen emits a bare
+#    `type: array` without the required `items` schema. A structural CRD must
+#    specify `items` for every array, so the generated CRD is rejected with:
+#      spec.validation.openAPIV3Schema.properties[configuration]
+#      .properties[maintenance_windows].items: Required value: must be specified
+
+file="${1:-"manifests/operatorconfiguration.crd.yaml"}"
+
+sed -i '/^[[:space:]]*maintenance_windows:$/{
+    # Capture the indentation
+    s/^\([[:space:]]*\)maintenance_windows:$/\1maintenance_windows:\n\1  items:\n\1    pattern: '\''^\\ *((Mon|Tue|Wed|Thu|Fri|Sat|Sun):(2[0-3]|[01]?\\d):([0-5]?\\d)|(2[0-3]|[01]?\\d):([0-5]?\\d))-((2[0-3]|[01]?\\d):([0-5]?\\d)|(2[0-3]|[01]?\\d):([0-5]?\\d))\\ *$'\''\n\1    type: string/
+}' "$file"
diff --git a/manifests/operatorconfiguration.crd.yaml b/manifests/operatorconfiguration.crd.yaml
index 5f347f2ac..67487ba1c 100644
--- a/manifests/operatorconfiguration.crd.yaml
+++ b/manifests/operatorconfiguration.crd.yaml
@@ -704,6 +704,9 @@ spec:
                     type: integer
                 type: object
               maintenance_windows:
+                items:
+                  pattern: '^\ *((Mon|Tue|Wed|Thu|Fri|Sat|Sun):(2[0-3]|[01]?\d):([0-5]?\d)|(2[0-3]|[01]?\d):([0-5]?\d))-((2[0-3]|[01]?\d):([0-5]?\d)|(2[0-3]|[01]?\d):([0-5]?\d))\ *$'
+                  type: string
                 type: array
               major_version_upgrade:
                 description: MajorVersionUpgradeConfiguration defines how to execute
diff --git a/pkg/apis/acid.zalan.do/v1/operatorconfiguration.crd.yaml b/pkg/apis/acid.zalan.do/v1/operatorconfiguration.crd.yaml
index 5f347f2ac..67487ba1c 100644
--- a/pkg/apis/acid.zalan.do/v1/operatorconfiguration.crd.yaml
+++ b/pkg/apis/acid.zalan.do/v1/operatorconfiguration.crd.yaml
@@ -704,6 +704,9 @@ spec:
                     type: integer
                 type: object
               maintenance_windows:
+                items:
+                  pattern: '^\ *((Mon|Tue|Wed|Thu|Fri|Sat|Sun):(2[0-3]|[01]?\d):([0-5]?\d)|(2[0-3]|[01]?\d):([0-5]?\d))-((2[0-3]|[01]?\d):([0-5]?\d)|(2[0-3]|[01]?\d):([0-5]?\d))\ *$'
+                  type: string
                 type: array
               major_version_upgrade:
                 description: MajorVersionUpgradeConfiguration defines how to execute