I took a look on the site itself. It turns out that while using extension Bonusfields the feature that belonged to the core extension of Gallery is overruled by Bonusfields (smart field).
While the galleryimagelist was never defined in Bonusfields itself on this site the export thinks it belongs to that extension.
As I cannot reproduce such a situation on my own site (and development of this wxrexport extension is also almost stopped) the solution for this problem is to just simply define the galleryimagelist in Bonusfields. Be aware that the name and internal name have to be "galleryimagelist" to match the existing definitions on the entries. After this the wxrexport gives no error message concerning this galleryimagelist any more.
BTW Galleryimagelist in Bonusfields gives you more functionality any way!
Quality is in the detail of things............
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