diff --git a/lib/AdminPages/Pages/Forms.php b/lib/AdminPages/Pages/Forms.php
index d1a9155645..19a9966810 100644
--- a/lib/AdminPages/Pages/Forms.php
+++ b/lib/AdminPages/Pages/Forms.php
@@ -64,9 +64,8 @@ class Forms {
       $data['site_url'] = $this->wp->siteUrl();
       $data['premium_plugin_active'] = License::getLicense();
       $data['current_wp_user_firstname'] = $this->wp->wpGetCurrentUser()->user_firstname;
-      $installedAtDateTime = new \DateTime($this->settings->get('installed_at'));
-      $data['installed_days_ago'] = (int)$installedAtDateTime->diff(new \DateTime())->format('%a');
-
+      $installedAtDiff = (new \DateTime($this->settings->get('installed_at')))->diff(new \DateTime());
+      $data['installed_days_ago'] = $installedAtDiff instanceof \DateInterval ? (int)$installedAtDiff->format('%a') : null;
       $data['display_nps_survey'] = true;
       $this->userFlags->set('display_new_form_editor_nps_survey', false);
     }
diff --git a/lib/AdminPages/Pages/Newsletters.php b/lib/AdminPages/Pages/Newsletters.php
index d7364ce4b2..76fd0c0c86 100644
--- a/lib/AdminPages/Pages/Newsletters.php
+++ b/lib/AdminPages/Pages/Newsletters.php
@@ -133,8 +133,9 @@ class Newsletters {
     $data['roles'] = $wp_roles->get_names(); // phpcs:ignore Squiz.NamingConventions.ValidVariableName.NotCamelCaps
     $data['roles']['mailpoet_all'] = $this->wp->__('In any WordPress role', 'mailpoet');
 
-    $installedAtDateTime = new \DateTime($data['settings']['installed_at']);
-    $data['installed_days_ago'] = (int)$installedAtDateTime->diff(new \DateTime())->format('%a');
+    $installedAtDiff = (new \DateTime($this->settings->get('installed_at')))->diff(new \DateTime());
+    $data['installed_days_ago'] = $installedAtDiff instanceof \DateInterval ? (int)$installedAtDiff->format('%a') : null;
+
     $data['subscribers_limit'] = $this->subscribersFeature->getSubscribersLimit();
     $data['subscribers_limit_reached'] = $this->subscribersFeature->check();
     $data['has_valid_api_key'] = $this->subscribersFeature->hasValidApiKey();
diff --git a/lib/Cron/Workers/ExportFilesCleanup.php b/lib/Cron/Workers/ExportFilesCleanup.php
index e64d187e28..2c7213d0d4 100644
--- a/lib/Cron/Workers/ExportFilesCleanup.php
+++ b/lib/Cron/Workers/ExportFilesCleanup.php
@@ -19,7 +19,7 @@ class ExportFilesCleanup extends SimpleWorker {
       $name = $file->getPathname();
       $created = $file->getMTime();
       $now = new Carbon();
-      if (Carbon::createFromTimestamp($created)->lessThan($now->subDays(self::DELETE_FILES_AFTER_X_DAYS))) {
+      if (Carbon::createFromTimestamp((int)$created)->lessThan($now->subDays(self::DELETE_FILES_AFTER_X_DAYS))) {
         unlink($name);
       };
     }
diff --git a/lib/Doctrine/MetadataCache.php b/lib/Doctrine/MetadataCache.php
index caa0bbab22..835a6ea22c 100644
--- a/lib/Doctrine/MetadataCache.php
+++ b/lib/Doctrine/MetadataCache.php
@@ -42,6 +42,9 @@ class MetadataCache extends CacheProvider {
     if ($fileExists && $this->isDevMode) {
       $classMetadata = unserialize((string)file_get_contents($filename));
       assert($classMetadata instanceof DoctrineClassMetadata || $classMetadata instanceof ValidatorClassMetadata);
+      if (!class_exists($classMetadata->name)) {
+        return false;
+      }
       try {
         $reflection = new ReflectionClass($classMetadata->name);
       } catch (ReflectionException $e) {
diff --git a/lib/DynamicSegments/RequirementsChecker.php b/lib/DynamicSegments/RequirementsChecker.php
index 2458c23db8..e30907e50a 100644
--- a/lib/DynamicSegments/RequirementsChecker.php
+++ b/lib/DynamicSegments/RequirementsChecker.php
@@ -34,7 +34,7 @@ class RequirementsChecker {
 
     $className = get_class($filter);
     $ref = new \ReflectionClass($className);
-    $constants = $ref->getConstants();
+    $constants = $ref->getConstants() ?? [];
     if (!array_key_exists('SEGMENT_TYPE', $constants)) {
       return true;
     }
diff --git a/lib/Entities/SettingEntity.php b/lib/Entities/SettingEntity.php
index de373c3bcd..68a1cc08c0 100644
--- a/lib/Entities/SettingEntity.php
+++ b/lib/Entities/SettingEntity.php
@@ -43,7 +43,7 @@ class SettingEntity {
 
   /** @return mixed */
   public function getValue() {
-    return is_serialized($this->value) ? unserialize($this->value) : $this->value;
+    return $this->value !== null && is_serialized($this->value) ? unserialize($this->value) : $this->value;
   }
 
   /** @param mixed $value */
diff --git a/lib/Features/FeatureFlagsController.php b/lib/Features/FeatureFlagsController.php
index cb0876be18..c156f71d97 100644
--- a/lib/Features/FeatureFlagsController.php
+++ b/lib/Features/FeatureFlagsController.php
@@ -36,9 +36,6 @@ class FeatureFlagsController {
       ),
       $flags
     );
-    if ($flagsMap === false) {
-      $flagsMap = [];
-    }
 
     $output = [];
     foreach ($this->featuresController->getDefaults() as $name => $default) {
diff --git a/lib/Mailer/Methods/AmazonSES.php b/lib/Mailer/Methods/AmazonSES.php
index 29003911d8..a9e1dee3c5 100644
--- a/lib/Mailer/Methods/AmazonSES.php
+++ b/lib/Mailer/Methods/AmazonSES.php
@@ -168,7 +168,7 @@ class AmazonSES {
         'Authorization' => $this->signRequest($body),
         'X-Amz-Date' => $this->date,
       ],
-      'body' => urldecode(http_build_query($body, null, '&')),
+      'body' => urldecode(http_build_query($body, '', '&')),
     ];
   }
 
@@ -209,7 +209,7 @@ class AmazonSES {
       'x-amz-date:' . $this->date,
       '',
       'host;x-amz-date',
-      hash($this->hashAlgorithm, urldecode(http_build_query($body, null, '&'))),
+      hash($this->hashAlgorithm, urldecode(http_build_query($body, '', '&'))),
     ]);
   }
 
diff --git a/lib/Mailer/Methods/SendGrid.php b/lib/Mailer/Methods/SendGrid.php
index 856f0bf09b..ab666f11f8 100644
--- a/lib/Mailer/Methods/SendGrid.php
+++ b/lib/Mailer/Methods/SendGrid.php
@@ -88,7 +88,7 @@ class SendGrid {
       'headers' => [
         'Authorization' => $this->auth(),
       ],
-      'body' => http_build_query($body, null, '&'),
+      'body' => http_build_query($body, '', '&'),
     ];
   }
 }
diff --git a/lib/Newsletter/Editor/PostContentManager.php b/lib/Newsletter/Editor/PostContentManager.php
index 54c3124c3b..47bdd7d0eb 100644
--- a/lib/Newsletter/Editor/PostContentManager.php
+++ b/lib/Newsletter/Editor/PostContentManager.php
@@ -60,6 +60,10 @@ class PostContentManager {
       $tagsNotBeingStripped = array_merge($tagsNotBeingStripped, ['<figure>', '<img>', '<h1>', '<h2>', '<h3>']);
     }
 
+    if (is_array($content)) {
+      $content = implode(' ', $content);
+    }
+
     $content = strip_tags($content, implode('', $tagsNotBeingStripped));
     if ($withPostClass) {
       $content = str_replace('<p', '<p class="' . self::WP_POST_CLASS . '"', WPFunctions::get()->wpautop($content));