RulesEvaluationException: Argument watchdog_link is missing. in RulesPlugin->setUpState() (line 648)

I have only the Watchdog Rules submodule enabled and created a very simple rule:

{ "rules_server_general_failure" : {
    "LABEL" : "Server-General Failure",
    "PLUGIN" : "reaction rule",
    "TAGS" : [ "error", "sg" ],
    "REQUIRES" : [ "rules", "watchdog_rules" ],
    "ON" : [ "watchdog_rules" ],
    "IF" : [
      { "data_is" : { "data" : [ "watchdog-type" ], "value" : "server_general" } }
    ],
    "DO" : [
      { "mail" : {
          "to" : "[site:mail]",
          "subject" : "Server-GENERAL User Registration Failed.",
          "message" : "Please check the Drupal reports for details."
        }
      }
    ]
  }
}

I get this message almost everywhere except for the admin pages. drush cc all also produces it.

When I disable the rule above, the error disappears. Upgrading Rules from rules-7.x-2.0 to rules-7.x-2.1 did not make a difference.