Acceptance Tests - problem to set current page


I use Yii 2.0.46 with functional testing and acceptance testing.
On functional test the following works:

On acceptance i get the following errors:
$I->amOnPage(['myctl/index']); -> ltrim() expects parameter 1 to be string, array given
$I->amOnPage('myctl/index'); -> shows the content of the default controller/action, not myctl/index
$I->amOnPage(Url::toRoute('myctl/index')); -> [yii\base\InvalidArgumentException] Unable to resolve the relative route: site/index. No active controller is available.

By the way: Url::toRoute('[/myctl/index']) works, but why not $I->amOnPage(['myctl/index']);, like on functional test?