[pierrec]

Many Christians in those regions were recently converted through violent means, see the Saxon Wars in the 8th century. It's to be expected that crypto-paganism would survive a long time after such events, but inherently difficult to find written traces: not only were these traditions oral, but now they had to be hidden.

One piece of evidence that comes to mind is recorded before the Stellinga uprising in the 9th century. Lothair, in his rebellion, got support of the local Stellinga (of doubtful religious allegiance) by promising them "if they should side with him, that he would let them have the same law in the future which their ancestors had observed when they were still worshiping idols." (From Scholz's Carolingian Chronicles, translations of Nithard's Histories).

https://en.wikipedia.org/wiki/Stellinga

Unfortunately, hints such as this are few and far between, but it still supports the idea that older cults may have been enduring. And of course, the last sentence about slipping off diseases, guilts and rebirth... that's just the little Wikipedia touch.