Shading strip on LS-40

Hello all,

I scan 35mm films with my LS-40, and I get a "shading strip" on the left border on most of my pictures.



It more or less visible depending of the picture (it's exposure?). It happens with B&W and neg color films.
It's not on the film: the strip is not on the pictures I previously scanned with a LS-2000.

I use SA-21, with VueScan on Linux.

A dirty mirror? A light leakage?
Any idea is welcome!

Clean the mirror.

I will try that. I'm a little afraid of damaging the mirror. I wouldn't want to manipulate it if not needed.

I thought the main consequence of a dirty mirror is getting blurry pictures.