MDL-11413 improved text cleaning (problem reported by Andreas Nolden); backported from HEAD

lib/weblib.php

@@ -1659,6 +1659,7 @@ function clean_text($text, $format=FORMAT_MOODLE) {
         /// Fix non standard entity notations
             $text = preg_replace('/(&#[0-9]+)(;?)/', "\\1;", $text);
             $text = preg_replace('/(&#x[0-9a-fA-F]+)(;?)/', "\\1;", $text);
+            $text = str_replace(':', ':', $text);
 
         /// Remove tags that are not allowed
             $text = strip_tags($text, $ALLOWED_TAGS);