Hi!
I already submitted a bug report on Loaders make full stacks for this, but thought I should add it here as well, just in case:
Placing a quality Loader with both mods enabled causes a non-recoverable error:

The mod Loaders make full stacks (1.1.2) caused a non-recoverable error.
Please report this error to the mod author.

Error while running event loaders-make-full-stacks::on_built_entity (ID 6)
__loaders-make-full-stacks__/control.lua:80: attempt to index global 'new_entity' (a nil value)
stack traceback:
    __loaders-make-full-stacks__/control.lua:80: in function 'replace'
    __loaders-make-full-stacks__/control.lua:160: in function <__loaders-make-full-stacks__/control.lua:155>

If this is an issue that can only be resolved by the Loader mod, then you may simply ignore this post

The same issue also occurs when using Lane Filtered Loaders, which was made by the same author: protocol_1903

These issue should be fixable by adding an optional dependency on more-quality-scaling to those loader mods.

I could theoretically add compatibility on my side, but would need to explicitly duplicate some logic from the loader mods, which seems like the less elegant solution.

The mod creator did sadly post about them not working on the mods, until they've made a combined mod. The last update they made was ~3 months ago

ALL ISSUES are being fixed in a unified loader mod, since in 2.0.65 Wube added a new loader property (respect insertion limit) and trying to manage 3 possibly interconnected mods is not worth the trouble. I will send a message here and update these mods accordingly once that combined loader mod is released.

You can unpack the lane-filtered-loaders and stacking-loaders mods in your mod directory (so the lane-filtered-loaders_1.1.17 folder sits in your mod directory), open the info.json of each and add "(?) more-quality-scaling", right after the "base >= 2.0.58", entry in the dependencies section. This should cause everything to work correctly.

edit: Those mods effectively being deprecated means that any compatibility I implemented would be worthless as soon as the combined loader configuration mod was made available. At that point, it might be a better use of my time to write that unified loader config mod myself, though if I were to do that, I should probably coordinate with protocol_1903 to prevent duplicating any effort.

Thanks a lot for the quick fix. Can confirm that it works like a charm.
I also messaged the mod creator on Discord, and maybe they'll implement this little work-around until they have the combined version released.

I'll be pushing the quick fix now, but as for an update on the combined loader mod: I just discovered that all of my progress was lost in a recent hard drive wipe (it was not, in fact, backed up to git) so I need to start from square one. I at least know how I want to do things so progress will be slightly faster to get back to where I was.

Thanks for letting us know. Sorry about the loss of data. I hope things go well for you.

https://mods.factorio.com/mod/loader-utils has been released