So, I've just updated to the latest 0.15.7 version to finally check if it'll start with the mods I've just updated.
Instead of this:

... I've got this:

... which is the same error message but in a different kind dialog.
I expected it to tell me at least which mod is causing the trouble, but I'm still clueless about which one it can be and where should I add the "icon_size" property, unless it's the "base" mod as the message says ¯\_(ツ)_/¯
Also, I think "base" shouldn't be even listed on the "Mods to disable" list, because disabling it essentially removes all in-game content.