When placing any higher quality locomotive inside a blueprint lab (https://mods.factorio.com/mod/blueprint-sandboxes), the game crashes. Modded locomotives do not crash and are improved by quality as expected.

The mod More Quality Scaling (1.2.2) caused a non-recoverable error.
Please report this error to the mod author.

Error while running event more-quality-scaling::on_built_entity (ID 6)
Invalid ItemID: expected LuaItemPrototype, LuaItemStack, LuaItem or string.
stack traceback:
[C]: in function 'insert'
more-quality-scaling/control.lua:107: in function 'restore_inventory'
more-quality-scaling/control.lua:217: in function <more-quality-scaling/control.lua:177>

So from a cursory look it seems like inv.get_contents() returns something that isn't a valid item that can be inv.insert()ed back into the corresponding inventory of the quality locomotive.

I'll look deeper into it when I have more time, but this might be an issue with blueprint-sandboxes.

I can't replicate this issue - I can place quality locomotives in the blueprint lab and don't see any crash. What other related mods did you have active?

It's Fill4Me: https://mods.factorio.com/mod/Fill4Me