Thanks for your answer.
As I just found out, both methods work:
lineEdit->setStyleSheet(styleSheet());
lineEdit->setStyleSheet(styleSheet());
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and
lineEdit->setStyleSheet("");
lineEdit->setStyleSheet("");
To copy to clipboard, switch view to plain text mode
But in my case the problem was somewhere else: the styleSheet-reset was overwritten by another setStyleSheet called by a signal-slot connection and thus it not change. *stupid*stupid* 
Kind regards,
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