This issue has occurred when NextCloud has been upgraded after deployment. Its source docker container may specify an older version as compared to the running instance. This discrepancy will cause the pod to fail to re-create or start as a new container as show below:

# Pod scheduling status yields 'Error'
kimconnect@k8sController:~$ k get pod
NAME                                              READY   STATUS    RESTARTS   AGE
clamav-0                                          1/1     Running   0          6d23h
collabora-collabora-code-69d74c979f-jp4p2         1/1     Running   0          6d19h
nextcloud-6cf9c65d85-42dx7                        1/2     Error     1          6s
nextcloud-db-postgresql-0                         1/1     Running   0          7d1h

# Further examination of the problem...
kimconnect@k8sController:~$ k describe pod nextcloud-6cf9c65d85-l9b99
Name:         nextcloud-6cf9c65d85-l9b99
Namespace:    default
Priority:     0
Node:         workder05/10.10.100.95
Start Time:   Fri, 20 Aug 2021 23:48:23 +0000
Labels:       app.kubernetes.io/component=app
              app.kubernetes.io/instance=nextcloud
              app.kubernetes.io/name=nextcloud
              pod-template-hash=6cf9c65d85
Annotations:  cni.projectcalico.org/podIP: 172.16.90.126/32
              cni.projectcalico.org/podIPs: 172.16.90.126/32
Status:       Running
IP:           172.16.90.126
IPs:
  IP:           172.16.90.126
Controlled By:  ReplicaSet/nextcloud-6cf9c65d85
Containers:
  nextcloud:
    Container ID:   docker://4c202d2155dea39739db815feae271fb8f14438f44092049f3d55c70fbf819c0
    Image:          nextcloud:stable-fpm
    Image ID:       docker-pullable://nextcloud@sha256:641b1dc10b681e1245c6f5d6d366fa1cd7e018ff787cf690c1aa372ddc108671
    Port:           <none>
    Host Port:      <none>
    State:          Waiting
      Reason:       CrashLoopBackOff
    Last State:     Terminated
      Reason:       Error
      Exit Code:    1
      Started:      Fri, 20 Aug 2021 23:54:03 +0000
      Finished:     Fri, 20 Aug 2021 23:54:03 +0000
    Ready:          False
    Restart Count:  6
    Environment:
      POSTGRES_HOST:              nextcloud-db-postgresql.default.svc.cluster.local
      POSTGRES_DB:                nextcloud
      POSTGRES_USER:              <set to the key 'db-username' in secret 'nextcloud-db'>      Optional: false
      POSTGRES_PASSWORD:          <set to the key 'db-password' in secret 'nextcloud-db'>      Optional: false
      NEXTCLOUD_ADMIN_USER:       <set to the key 'nextcloud-username' in secret 'nextcloud'>  Optional: false
      NEXTCLOUD_ADMIN_PASSWORD:   <set to the key 'nextcloud-password' in secret 'nextcloud'>  Optional: false
      NEXTCLOUD_TRUSTED_DOMAINS:  kimconnect.com
      NEXTCLOUD_DATA_DIR:         /var/www/html/data
    Mounts:
      /usr/local/etc/php-fpm.d/memory_limit from nextcloud-phpconfig (rw,path="memory_limit")
      /usr/local/etc/php-fpm.d/post_max_size from nextcloud-phpconfig (rw,path="post_max_size")
      /usr/local/etc/php-fpm.d/upload_max_filesize from nextcloud-phpconfig (rw,path="upload_max_filesize")
      /usr/local/etc/php-fpm.d/upload_max_size from nextcloud-phpconfig (rw,path="upload_max_size")
      /var/run/secrets/kubernetes.io/serviceaccount from default-token-bdhxv (ro)
      /var/www/ from nextcloud-data (rw,path="root")
      /var/www/html from nextcloud-data (rw,path="html")
      /var/www/html/config from nextcloud-data (rw,path="config")
      /var/www/html/custom_apps from nextcloud-data (rw,path="custom_apps")
      /var/www/html/data from nextcloud-data (rw,path="data")
      /var/www/html/themes from nextcloud-data (rw,path="themes")
      /var/www/tmp from nextcloud-data (rw,path="tmp")
  nextcloud-nginx:
    Container ID:   docker://1fae573d1a0591058ad55f939b4762f01c7a5f6e7275d2348ff1bd287e077fe5
    Image:          nginx:alpine
    Image ID:       docker-pullable://nginx@sha256:e20c21e530f914fb6a95a755924b1cbf71f039372e94ac5ddcf8c3b386a44615
    Port:           80/TCP
    Host Port:      0/TCP
    State:          Running
      Started:      Fri, 20 Aug 2021 23:48:26 +0000
    Ready:          True
    Restart Count:  0
    Environment:    <none>
    Mounts:
      /etc/nginx/nginx.conf from nextcloud-nginx-config (rw,path="nginx.conf")
      /var/run/secrets/kubernetes.io/serviceaccount from default-token-bdhxv (ro)
      /var/www/ from nextcloud-data (rw,path="root")
      /var/www/html from nextcloud-data (rw,path="html")
      /var/www/html/config from nextcloud-data (rw,path="config")
      /var/www/html/custom_apps from nextcloud-data (rw,path="custom_apps")
      /var/www/html/data from nextcloud-data (rw,path="data")
      /var/www/html/themes from nextcloud-data (rw,path="themes")
      /var/www/tmp from nextcloud-data (rw,path="tmp")
Conditions:
  Type              Status
  Initialized       True
  Ready             False
  ContainersReady   False
  PodScheduled      True
Volumes:
  nextcloud-data:
    Type:       PersistentVolumeClaim (a reference to a PersistentVolumeClaim in the same namespace)
    ClaimName:  nextcloud-claim
    ReadOnly:   false
  nextcloud-phpconfig:
    Type:      ConfigMap (a volume populated by a ConfigMap)
    Name:      nextcloud-phpconfig
    Optional:  false
  nextcloud-nginx-config:
    Type:      ConfigMap (a volume populated by a ConfigMap)
    Name:      nextcloud-nginxconfig
    Optional:  false
  default-token-bdhxv:
    Type:        Secret (a volume populated by a Secret)
    SecretName:  default-token-bdhxv
    Optional:    false
QoS Class:       BestEffort
Node-Selectors:  <none>
Tolerations:     node.kubernetes.io/not-ready:NoExecute op=Exists for 300s
                 node.kubernetes.io/unreachable:NoExecute op=Exists for 300s
Events:
  Type     Reason     Age                  From               Message
  ----     ------     ----                 ----               -------
  Normal   Scheduled  10m                  default-scheduler  Successfully assigned default/nextcloud-6cf9c65d85-l9b99 to linux05
  Normal   Pulled     10m                  kubelet            Container image "nginx:alpine" already present on machine
  Normal   Created    10m                  kubelet            Created container nextcloud-nginx
  Normal   Started    10m                  kubelet            Started container nextcloud-nginx
  Normal   Created    9m47s (x4 over 10m)  kubelet            Created container nextcloud
  Normal   Started    9m46s (x4 over 10m)  kubelet            Started container nextcloud
  Normal   Pulled     8m55s (x5 over 10m)  kubelet            Container image "nextcloud:stable-fpm" already present on machine
  Warning  BackOff    18s (x51 over 10m)   kubelet            Back-off restarting failed container

# Checking the logs
kimconnect@k8sController:~$ k logs nextcloud-6cf9c65d85-l9b99 nextcloud
Can't start Nextcloud because the version of the data (21.0.4.1) is higher than the docker image version (20.0.8.1) and downgrading is not supported. Are you sure you have pulled the newest image version?

Solution:

# a. Create a backup copy of version.php
  sudo mount $nfsServer:/volume1/nextcloud /mnt/nextcloud
  cd /mnt/nextcloud/html
  cp version.php version.php.bak

# b. Edit the version.php file with this content
  vim version.php
########
# <?php
# $OC_Version = array(21,0,4,1); # change this value to array(20,0,8,1)
# $OC_VersionString = '21.0.4'; # change this value to '20.0.8'
# $OC_Edition = '';
# $OC_Channel = 'stable';
# $OC_VersionCanBeUpgradedFrom = array (
#   'nextcloud' =>
#   array (
#     '20.0' => true,
#     '21.0' => true,
#   ),
#   'owncloud' =>
#   array (
#     '10.5' => true,
#   ),
# );
# $OC_Build = '2021-08-03T15:44:43+00:00 c52fea0b16690b492f6c4175e1ae71d488936244';
# $vendor = 'nextcloud';
########

# c. Recreate the failed pod and verify that it's in 'running status'

kimconnect@k8sController:~$ k delete pod nextcloud-6cf9c65d85-l9b99
pod "nextcloud-6cf9c65d85-l9b99" deleted
kimconnect@k8sController:~$ k get pod
NAME                                              READY   STATUS    RESTARTS   AGE
clamav-0                                          1/1     Running   0          6d23h
collabora-collabora-code-69d74c979f-jp4p2         1/1     Running   0          6d19h
nextcloud-6cf9c65d85-dmg2s                        2/2     Running   0          17s
nextcloud-db-postgresql-0                         1/1     Running   0          7d1h

# d. Revert changes to version.php

cd /mnt/nextcloud/html
mv version.php version.php.old
mv version.php.bak version.php