diff --git a/lib/Activity/ActivityManager.php b/lib/Activity/ActivityManager.php
index 9fc7286fa..4a74e2d9d 100644
--- a/lib/Activity/ActivityManager.php
+++ b/lib/Activity/ActivityManager.php
@@ -367,7 +367,15 @@ class ActivityManager {
 			case self::SUBJECT_CARD_USER_ASSIGN:
 			case self::SUBJECT_CARD_USER_UNASSIGN:
 				$subjectParams = $this->findDetailsForCard($entity->getId(), $subject);
-				break;
+
+				if (isset($additionalParams['after']) && $additionalParams['after'] instanceof \DateTimeInterface) {
+					$additionalParams['after'] = $additionalParams['after']->format('c');
+				}
+				if (isset($additionalParams['before']) && $additionalParams['before'] instanceof \DateTimeInterface) {
+					$additionalParams['before'] = $additionalParams['before']->format('c');
+				}
+
+			break;
 			case self::SUBJECT_ATTACHMENT_CREATE:
 			case self::SUBJECT_ATTACHMENT_UPDATE:
 			case self::SUBJECT_ATTACHMENT_DELETE:
diff --git a/lib/Activity/DeckProvider.php b/lib/Activity/DeckProvider.php
index 4235816e5..a2a5c9d8c 100644
--- a/lib/Activity/DeckProvider.php
+++ b/lib/Activity/DeckProvider.php
@@ -312,12 +312,19 @@ class DeckProvider implements IProvider {
 			$userLanguage = $this->config->getUserValue($event->getAuthor(), 'core', 'lang', $this->l10nFactory->findLanguage());
 			$userLocale = $this->config->getUserValue($event->getAuthor(), 'core', 'locale', $this->l10nFactory->findLocale());
 			$l10n = $this->l10nFactory->get('deck', $userLanguage, $userLocale);
-			$date = new \DateTime($subjectParams['after']);
-			$date->setTimezone(new \DateTimeZone(\date_default_timezone_get()));
+			if (is_array($subjectParams['after'])) {
+				// Unluckily there was a time when we stored jsonSerialized date objects in the database
+				// Broken in 1.8.0 and fixed again in 1.8.1
+				$date = new \DateTime($subjectParams['after']['date']);
+				$date->setTimezone(new \DateTimeZone(\date_default_timezone_get()));
+			} else {
+				$date = new \DateTime($subjectParams['after']);
+				$date->setTimezone(new \DateTimeZone(\date_default_timezone_get()));
+			}
 			$params['after'] = [
 				'type' => 'highlight',
 				'id' => 'dt:' . $subjectParams['after'],
-				'name' => $l10n->l('datetime', $date)
+				'name' => $l10n->l('datetime', $date),
 			];
 		}
 		return $params;