Member Posts in Private Channel fail to send

Turns out it wasn't that, however, that did help lead me in the right direction: our General channel within the team had "Only owners can post messages" (which is a setting we want for General). I flipped it back to "Anyone can post messages" to see if that would affect my private channels, but unfortunately it didn't. I still couldn't post.

However, I left the General channel set to "Anyone can post messages", and I created another new private test channel, and this time it worked. I was able to create new posts, as a member, within my private channel just fine. I tested with a guest account too, and that also worked. I then flipped the General channel back to "Only owners can post messages", and I was still able to post messages in my test private channel.

So, it seems like, upon creation of the channel, the General channel's setting is mirrored into the new private channel, only there appears to be no obvious way to tweak it within the private channel after it's been created.

I'll open a ticket with Microsoft to assist with our already-active channels, but at least I have this workaround for new channels. Thanks for leading me in the right direction!