diff --git a/CHANGELOG.md b/CHANGELOG.md
index 6651ae978fc..732d381814c 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -62,6 +62,7 @@
 - Fix double scrollbar when composing a mail with many plain text lines (#7760)
 - Fix decoding mail parts with multiple base64-encoded text blocks (#9290)
 - Fix bug where some messages could get malformed in an import from a MBOX file (#9510)
+- Fix invalid line break characters in multi-line text in Sieve scripts (#9543)
 
 ## Release 1.6.7
 
diff --git a/plugins/managesieve/Changelog b/plugins/managesieve/Changelog
index a7745951c34..f26d7058641 100644
--- a/plugins/managesieve/Changelog
+++ b/plugins/managesieve/Changelog
@@ -1,3 +1,4 @@
+- Fix invalid line break characters in multi-line text in Sieve scripts (#9543)
 - Fix javascript error when relational or spamtest extension is not enabled (#9139)
 - Support an array in managesieve_host option (#9447)
 
diff --git a/plugins/managesieve/lib/Roundcube/rcube_sieve_script.php b/plugins/managesieve/lib/Roundcube/rcube_sieve_script.php
index b82e24876df..7028b7f96e8 100644
--- a/plugins/managesieve/lib/Roundcube/rcube_sieve_script.php
+++ b/plugins/managesieve/lib/Roundcube/rcube_sieve_script.php
@@ -1204,6 +1204,8 @@ public static function escape_string($str)
             $str = array_pop($str);
         }
 
+        $str = (string) $str;
+
         // multi-line string
         if (preg_match('/[\r\n\0]/', $str)) {
             return sprintf("text:\r\n%s\r\n.\r\n", self::escape_multiline_string($str));
@@ -1222,7 +1224,7 @@ public static function escape_string($str)
      */
     public static function escape_multiline_string($str)
     {
-        $str = preg_split('/(\r?\n)/', $str, -1, \PREG_SPLIT_DELIM_CAPTURE);
+        $str = preg_split('/\r?\n/', $str);
 
         foreach ($str as $idx => $line) {
             // dot-stuffing
@@ -1231,7 +1233,7 @@ public static function escape_multiline_string($str)
             }
         }
 
-        return implode('', $str);
+        return implode("\r\n", $str);
     }
 
     /**
diff --git a/plugins/managesieve/tests/ManagesieveScriptTest.php b/plugins/managesieve/tests/ManagesieveScriptTest.php
index ab65d145559..a793b4d9377 100644
--- a/plugins/managesieve/tests/ManagesieveScriptTest.php
+++ b/plugins/managesieve/tests/ManagesieveScriptTest.php
@@ -84,4 +84,21 @@ public function test_tokenizer($num, $input, $output)
 
         $this->assertSame(trim($output), trim($res));
     }
+
+    public function test_escape_string()
+    {
+        $output = \rcube_sieve_script::escape_string([]);
+        $this->assertSame('""', $output);
+        $output = \rcube_sieve_script::escape_string(['"']);
+        $this->assertSame('"\""', $output);
+        $output = \rcube_sieve_script::escape_string('\\a');
+        $this->assertSame('"\\\\a"', $output);
+        $output = \rcube_sieve_script::escape_string(['"', 'b']);
+        $this->assertSame('["\"","b"]', $output);
+
+        // Multiline text
+        $input = "line1\r\nline2\n.line3\r\nline4";
+        $output = \rcube_sieve_script::escape_string($input);
+        $this->assertSame("text:\r\nline1\r\nline2\r\n..line3\r\nline4\r\n.\r\n", $output);
+    }
 }