random_bytes() can throw an exception in some cases, since PHP 7.0.0rc3

program/lib/Roundcube/rcube_utils.php

@@ -1101,8 +1101,12 @@ class rcube_utils
     {
         // Use PHP7 true random generator
         if (function_exists('random_bytes')) {
+            // random_bytes() can throw an exception in some cases
+            try {
                 $random = @random_bytes($length);
             }
+            catch (Exception $e) {}
+        }
 
         if (!$random) {
             $random = openssl_random_pseudo_bytes($length);