diff --git a/de.systopia.civiproxy/CRM/Civiproxy/Mailer.php b/de.systopia.civiproxy/CRM/Civiproxy/Mailer.php
index ce6e4ac7..90284a9b 100644
--- a/de.systopia.civiproxy/CRM/Civiproxy/Mailer.php
+++ b/de.systopia.civiproxy/CRM/Civiproxy/Mailer.php
@@ -62,7 +62,9 @@ static function mendURLs(&$value) {
     $value = preg_replace("#{$system_base}sites/default/files/civicrm/persist/#i",      $proxy_base.'/file.php?id=',    $value);
     $value = preg_replace("#{$system_base}civicrm/mosaico/img\?src=#i",                 $proxy_base.'/mosaico.php?id=', $value);
     if ($mosaico->isMosaicoInstalled()) {
-      $value = preg_replace("#({$mosaico->getMosaicoExtensionUrl()}/packages/mosaico/templates/)(\S*)([\"'])#i", $proxy_base . '/mosaico.php?template_url=$2$3', $value);
+      $value = preg_replace_callback("#({$mosaico->getMosaicoExtensionUrl()}/packages/mosaico/templates/)(\S*)([\"'])#i", function($matches) use ($proxy_base) {
+        return $proxy_base . '/mosaico.php?template_url=' . urlencode($matches[2]) . $matches[3];
+      }, $value);
     }
 
     // Mailing related functions