This issue is described here (in german): Externer Speicher immer Rotes Schild.
Here original post: https://github.com/nextcloud/server/issues/17846
In SMB.php after following entries:
(new DefinitionParameter('timeout', $l->t('Timeout')))
->setType(DefinitionParameter::VALUE_HIDDEN)
add following entry:
->setFlag(DefinitionParameter::FLAG_OPTIONAL),