There are several errors when submitting forum topics:
Fatal error: Allowed memory size of 536870912 bytes exhausted (tried to allocate 65552 bytes) in /home/forum/repo/www/core/includes/bootstrap.inc on line 2593
This is a security precaution. It's not recommended and it's not documented in the settings.php file, but you can set allow_insecure_uploads to be TRUE if you wish to allow uploading files like...
Comments
This is probably related to:
https://forum.backdropcms.org/forum/forum-preview-throws-error and/or
https://github.com/backdrop-ops/forum.backdropcms.org/issues/67
Thanks for reporting the problem.