Look for element instead of text to support potential custom error page.

tests/codeception/e2e/app/en/LanguagesEnCest.php

@@ -23,6 +23,6 @@ class LanguagesEnCest
         $I->seeElement('.site-error');
 
         $I->amOnPage('/en-us');
-        $I->see('Not Found');
+        $I->seeElement('.site-error');
     }
 }

tests/codeception/e2e/app/en/SignupCest.php

@@ -24,7 +24,7 @@ class SignupCest
             $I->amOnPage('/user/login');
             $I->dontSeeLink("Don't have an account? Sign up!", '/en/user/register');
             $I->amOnPage('/user/register');
-            $I->see('Not Found (#404)');
+            $I->seeElement('.site-error');
         }
     }
 }