@manton I’ve also just noticed this issue today. I tried to reply to a few folks on micro.blog using the macOS app and it crashes immediately. Posting to micro.blog works fine though. Using the non-beta iOS app gives no indication of error, but the replies never appear in my timeline, nor do they get to the recipient.
UPDATE: I just deleted the blog that was tied to my micro.blog account and that seems to have fixed the replies.