What does "FailedMount: no Pending workload pods for volume" mean?

In a k3s cluster (with multiple control-plane nodes) and with Rancher Longhorn installed, I am observing the following warning for pods that have a pvc with the default longhorn storage class (output of kubectl get events, wrapped for better readability):

LAST SEEN   TYPE      REASON        OBJECT                      
113s        Warning   FailedMount   pod/grafana-6756f6587b-rv2xj

   MESSAGE
   MountVolume.SetUp failed for volume "pvc-7b6d12e3-132d-4af1-99c0-920ac5af0687" :
     rpc error:
       code = Aborted
       desc = no Pending workload pods for volume pvc-7b6d12e3-132d-4af1-99c0-920ac5af0687
              to be mounted: map[Running:[grafana-6756f6587b-rv2xj]]

What does this recurring warning mean and what action is required to fix it?

At least from what I can see in the pod's container, the volume has been mounted. I already tried restarting the pod and also recreating the pod, but the warning still persists. Also the longhorn dashboard does not show any problems.

See below for the pvc and deployment resource definitions:

---
apiVersion: v1
kind: PersistentVolumeClaim
metadata:
  name: grafana-pvc
spec:
  accessModes:
    - ReadWriteOnce
  volumeMode: Filesystem
  storageClassName: longhorn
  resources:
    requests:
      storage: 1Gi
---
apiVersion: apps/v1
kind: Deployment
metadata:
  name: grafana
  namespace: monitoring
  labels:
    app: grafana
spec:
  selector:
    matchLabels:
      app: grafana
  template:
    metadata:
      labels:
        app: grafana
    spec:
      securityContext:
        fsGroup: 472
        supplementalGroups:
          - 0
      containers:
        - name: grafana
          image: grafana/grafana:latest
          imagePullPolicy: IfNotPresent
          ports:
            - containerPort: 3000
              name: http-grafana
              protocol: TCP
          readinessProbe:
            failureThreshold: 3
            httpGet:
              path: /robots.txt
              port: 3000
              scheme: HTTP
            initialDelaySeconds: 10
            periodSeconds: 30
            successThreshold: 1
            timeoutSeconds: 2
          livenessProbe:
            failureThreshold: 3
            initialDelaySeconds: 30
            periodSeconds: 10
            successThreshold: 1
            tcpSocket:
              port: 3000
            timeoutSeconds: 1
          resources:
            requests:
              cpu: 250m
              memory: 750Mi
          volumeMounts:
            - mountPath: /var/lib/grafana
              name: grafana-pv
      volumes:
        - name: grafana-pv
          persistentVolumeClaim:
            claimName: grafana-pvc