When a crusher is running the Grass Seed Box recipe (turning grass into seeds and cellulose), it stops after every cycle, saying that its output is full. This means you must have an excess of inserters taking out the products in order to get one to run at full speed.

Are you using the boxed version of the recipe? That helps a lot. It's a fast recipe that produces 20 output. Inserters have a finite throughput, so if the speed of the recipe exceeds that it's possible to need multiple inserters.

Yeah, it's the boxed version, but like I said, even a single completed recipe causes the machine to stop until the inserters have removed everything.

Thats ... normal? I mean, when you have backed up product, machines stop until you remove it.

Use bigger inserters. Stack inserters. Belt loaders. Something like that. Or just use two inserters. Even in vanilla Factorio sometimes it is useful to use two inserters instead of one.

It is absolutely not normal. Normally, stack size is what clogs a recipe so that it stops. In this case, even with 2 seed boxes left in the machine it won't make any more.

I increased stack sizes for grass seed and cellulose in the latest update, which should help with this issue.