Drills do not receive a bonus to the number of modules. If I try to select a quality drill, it gives an error on another mod. When I disable the increase in slots for modules, the error disappears.

Here is the error log.

Error while running event quality-lib::on_gui_opened (ID 99)
Unknown item name: I-electric-mining-drill
stack traceback:
[C]: in function 'set_stack'
quality-lib/control.lua:146: in function <quality-lib/control.lua:130>

Here is a save with mods.
https://drive.google.com/drive/folders/1HuD8saJ6IKYAPWVuod8osrSB-4uEJW3U?usp=sharing

I see, so some kind of issue with custom entities. I will have a peek at what I can do, I have just been busy with other things at the moment so I cannot promise I can address this immediately. I will definitely come back to this soon so I can try to work something out.

Just to get the ball rolling, try to isolate the mod which is having the conflict. I took a quick look at your save file and there are far too many addons to dig through to find it myself. First I would find all the addons which have a requirement for quality-lib and only enable those. Then start removing them until you find the one that is having issues. This would help a lot to assist in debugging this issue. If you can follow up with the conflicting mods then I can do a better job at finding a solution when I get back around to this.

I have a problem that I can no longer cause this bug. Perhaps I disabled the mod that was in conflict with. Most likely it is this mod since it has a "stack set" setting. Now the drills simply do not have a bonus to the slots. I alternately disabled each mod with quality, but there were no changes.

https://mods.factorio.com/mod/quality-miners

Thank you for doing some of the legwork on tracking this down, and it seems that the previous bugfix broke the functionality of the module slot bonus being applied. I assumed that it would work similarly to the assembly machines, which had no issues with the same logic of checking both allowed effects as well as the module slots.

To simplify the logic we now just check for module slots on the entity, which comes with the assumption that it will have allowed effects as well. This fixes the issue and allows for the entity to have its scaled module slots.

This fix is now on the github and will be in the next release soon.