It means a feverish cloud and is just a metaphor.
C’est une forêt vierge où les mots d’amour sont maudits
de fièvre entre la Sologne et Paris
[The place where I love you]
It’s an untouched forest where words
of love are damned
A feverish cloud between the Sologne region
The structure of the entire song is built around the idea of The place where I love you is: a series of sentences describing that place. The Sologne is the place where all this bucolic ardor takes place…..The song is both happy and sad, as love typically is.
In short, the Sologne is a picture post card of ponds, forests and castles, as one site on the Internet puts it. The suggestion is that the lovers leave Paris and head to Sologne to be together and all along the route there is a feverish cloud over them as they rush to their romantic destination where this love is “realized”.
Beside, it might be a poetic metaphor in these lyrics, but it is not at all a “French expression”: it was used nowhere else.