a week after doing updates, i just updated right now, factorio from 0.18.10 to 0.18.14, and this mod to 0.3.16), and got a long list of mods to disable when the game restarted. it seems that 23 (including the mining drones) of my many mods had this same error :

Failed to load mods:
Error while loading gui-style prototype "default" (gui-style): Parent style not found: at_button_slot.

what has changed that so many mods show the same error? a bug in the vanilla game, or more strict checks that now find problems in so many mods, or maybe some conflicts from interactions of some mods with each other ?

TLDR : all these mods i mentioned (including this one) now work again,
causing no problems when starting factorio, after some were updated and ALL are enabled again.

more detailed: after disabling all those 23 mods and restarting factorio successfully, i could read the vanilla changelog and saw that the gui was changed (in 0.18.13) during the above mentioned updates. a few hours later, 4 of those 23 mods (and 15 others that were only installed and not enabled) were updated and i tried again: now, even after enabling ALL those 23 mods again, factorio starts properly again!

thus i conclude (guess :-) that the prime reason for the problem was some mod's reaction to the gui changes in vanilla, and secondary that that mod somehow made all those other mods show problems with the prototypes in some kind of chain reaction.
just for info in case anybody wants to investigate further: these four mods were AutoTrash (5.0.2 to 5.1.0), Industrial Display Plates (1.0.4 to 1.0.5), LTN Combinator (0.6.0 to 0.6.1), and helmod (0.10.10 to 0.10.11).