The conveyor receptacle is not accepting material from the conveyor rail, causing the conveyor rail to back up. As you can see, there is 200 kg of dirt ready to enter the conveyor rail.
Wait, I think the question/bug is "how did 200 kg enter a rail box when the rail boxes are limited to 20 kg!?!?"
By the way, love the game! So glad we're entering a cleanup phase, I hope I can be a useful part of that!
See attached save file. Rail box (200 kg) is above the normal value for rail boxes (which is normally 20 kg), and the receptacle cannot hold more than 100 kg