Blog posts not saving

I'm having problems when trying to save a new blog post. When I hit 'Create draft' it goes back to the top of the page and clears the fields. If I then go to the post listing bit of the CMS there's no post. I'm the primary admin, so it shouldn't be a permissions thing and it was only the 16th of October that I published the last post successfully. I was running 3.1.2 at that point (I updated on the 26th of August) and am now on 3.1.3 so it might be something in the newest version.

Ordinary content seems to be working fine – just the blog.

Here's my diagnostics:

Perch Runway: 3.1.3, PHP: 7.2.8, MySQL: mysqlnd 5.0.12-dev - 20150407 - $Id: 38fea24f2847fa7519001be390c98ae0acafe387 $, with PDO
Server OS: Darwin, cgi-fcgi
Installed apps: content (3.1.3), assets (3.1.3), categories (3.1.3), perch_blog (5.6.1), perch_forms (1.12)
App runtimes: <?php $apps_list = [ 'perch_blog', 'perch_forms', ];
PERCH_LOGINPATH: /cms
PERCH_PATH: /Users/martin/Sites/tempertemper.net/web/cms
PERCH_CORE: /Users/martin/Sites/tempertemper.net/web/cms/core
PERCH_RESFILEPATH: /Users/martin/Sites/tempertemper.net/web/cms/resources
Image manipulation: GD
PHP limits: Max upload 32M, Max POST 8M, Memory: 128M, Total max file upload: 8M
F1: 3b606135b33e6a102526838f4152a807
Resource folder writeable: Yes
HTTP_HOST: tempertemper.local
DOCUMENT_ROOT: /Users/martin/Sites/tempertemper.net/web
REQUEST_URI: /cms/core/settings/diagnostics/
SCRIPT_NAME: /cms/core/settings/diagnostics/index.php

Thanks for taking a look :)