I see that in addition to the nginx hack for request size >>23 there is also one for paragraph length. This is not a problem for those of us not running our own instances, but if these are private then it seems the docker people do not have them available.
https://fossil.textboard.org/docker-schemebbs-nginx/artifact/6f4a819fea1ea1fa
There also doesn't seem to be a good reason to disallow posts filled up entirely with one paragraph.