I use stacked coal as fuel for furnaces, but after a short time all inserters get blocked while trying to insert more stacked coal items as allowed. I removed the fuel, but short later, some inserters got stucked again.

Screenshot:
https://s17.directupload.net/images/190620/4pirzhoz.png

At this pciture there are already 5 coal items in the furnace and the inserter carries another coal item.
There are hundert more furnaces to the right, all of them got in trouble - zero production.

Sys specs:
Factorio 0.17.50
Beltboxes 2.2.0 - stack size changed from 5 (default) to 10 in menu settings.
+ other mods

EDIT:
I have reproduced the bug with a fresh clean game (only 30 seconds gameplay) without any other mods but beltboxes.
The last furnace in the row is blocked already at this savegame.

Upload with savegame + mod directory:
https://www.file-upload.net/download-13640894/FurnaceOverloading.zip.html

This is.. really weird. The stacked coal is very much a 'normal' fuel item being handled by the factorio engine just like normal coal would, it just happens to have a higher fuel value and lower stack size. You might've found an unintended behavior in the base game around how it's enforcing fuel stack sizes on furnances.

It seems that this is due to the vanilla inserter behavior - they're grabbing 2 when there's room for 1 in the furnace, then holding the other; fuel items with stack sizes of between 2 and 5 can potentially jam up like this :(