Wartungsmodus (BE) + Preview + ESI Renderer = Error 503

Mein Besucher Modul ist vom Typ renderer: esi
Funktioniert soweit gut, auch im Preview Modus.
Wenn aber der Wartungsmodus noch dazu geschaltet ist über das Backend, dann kommt es zur Meldung:

Code:

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Uncaught PHP Exception RuntimeException: "Error when rendering "https://contao53.contao5dev/preview.php/_fragment?_hash=x6dzWhlyUOmL24yxwn87Cg2m7agVCKmC5DPaph6qwwA%3D&_path=_scope%3Dfrontend%26moduleModel%3D68%26section%3Dleft%26pageModel%3D48%26_format%3Dhtml%26_locale%3Den%26_controller%3Dcontao.frontend_module.visitors"
(Status code is 503)." at AbstractSurrogate.php line 92

{
"exception": "[object] (RuntimeException(code: 0): Error when rendering "https://contao53.contao5dev/preview.php/_fragment?_hash=x6dzWhlyUOmL24yxwn87Cg2m7agVCKmC5DPaph6qwwA%3D&_path=_scope%3Dfrontend%26moduleModel%3D68%26section%3Dleft%26pageModel%3D48%26_format%3Dhtml%26_locale%3Den%26_controller%3Dcontao.frontend_module.visitors"
(Status code is 503). at /daten/contao5go/contao53.contao5dev/vendor/symfony/http-kernel/HttpCache/AbstractSurrogate.php:92)"
}
{
"request_uri": "https://contao53.contao5dev/preview.php/_fragment?_hash=TS0ysAJdCH0DXqYG13E7%2FlD6plyqM7hlZuLnEttiZ50%3D&_path=insertTag%3D%257B%257Bcache_botstatistics%253A%253A70%253A%253Acount%253A%253A%257D%257D%26_scope%3Dfrontend%26_format%3Dhtml%26_locale%3Den%26_controller%3DContao%255CCoreBundle%255CController%255CInsertTagsController%253A%253ArenderAction&clientCache=0&pageId=71&request=%2Fpreview.php%2F_fragment%3F_hash%3Dx6dzWhlyUOmL24yxwn87Cg2m7agVCKmC5DPaph6qwwA%253D%26_path%3D_scope%253Dfrontend%2526moduleModel%253D68%2526section%253Dleft%2526pageModel%253D48%2526_format%253Dhtml%2526_locale%253Den%2526_controller%253Dcontao.frontend_module.visitors",
"request_method": "GET"
}

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Schalte ich dann noch Debug dazu ist der Fehler nicht mehr da.

Ist nicht nur bei mir so: Ticket