We bought a new Bosch mixer a few months ago. Previous one died after few years but had the same problem as the new one: as soon as a bit of water gets onto the shredder head, it starts spitting rusty liquid into the shredding can.
The head has "no water" sign on it but it's impossible to avoid all water contact since drops of food get on it during the operation and have to be cleaned somehow.
The head can be basically pulled apart as it has no screws or glue, start on the side and then pull the internal bottom gear away.
The problem is clear: metal collet hiding inside the bottom gear is completely covered in rust. It's not staneless and not even lubricated.
Overnight vinegar bath and a lot of paper towels mostly fix that.
Assembling back with a lot of food-safe silicone grease.
Covering the metal with grease from all sides to delay rusting next time water gets on it.
Final assembly requires pushing down the top pin.
Filling in the side wall damage with grease and wiping the excess away with a paper towel to prevent easy water access. Back to the onion shredding business 😃