Weird xsession warning message

[Brian49]

Every time I launch a new X session, this line appears in my .xsession-errors file:

"(xfdesktop:2234): WARNING **: Thumbnailer failed calling GetFlavors"

I don't have any thumbnailers installed, and I have no idea what GetFlavors is. A web search brings no useable information. Can anyone here very kindly point me in the right direction, please? I should add that there is no visible sign of trouble apart from this line in the file. Many thanks.