Folder not being excluded

[mauricev]

I have a file share with numerous folders named _YU6L1~X (where do they come from, anyway?). I tried to exclude them in FFS, but it didn't work. You can see the exclusion and the folder showing up in the destination here

[Plerry]

I have a file share with numerous folders named _YU6L1~X

So, it is not just a single folder named _YU6L1~X but multiple such folders spread over the folder-tree of your left and right base location.
Your Exclude rule only excludes any _YU6L1~X folder in the root of the left and right base location, not in any of its subdirectories.
Try excluding *\_YU6L1~X\ if you want to exclude all _YU6L1~X (sub)directories.