chicks get b*tts clogged with poop
Pasty butt is not uncommon in chicks, especially those raised in a brooder situation - the exact cause(s) is often subject of debate, but factors that tend to cause an increase in the occurrence are heat (too high), feed (folks that feed fermented, soaked, etc report less frequent issues), stress, dehydration (going back to the use of "wet" feed in soaked or fermented form being something that can decrease it), etc. There are a lot of different approaches folks use that seem to help them decrease the issue - the key is going to be identifying the one(s) you want to employ.
In the meantime, keep checking and cleaning vents daily - use lots of warm water to soak the crust loose and be careful not to pull as you can easily tear the delicate skin. Once clean and dry you can apply a little petroleum jelly, etc to prevent further sticking of waste to the down or even just trim a bit of down to help decrease the available surface for waste to initially cling to and create the next build up.