Should be fixed now. The problem is as follows:
* By default, most system files for the forum and front page software (vB and WP) are owned by the www-data user, e.g. the web server software itself.
* This has the benefit that for example Wordpress can update itself.
* This has the downside that, if there is a security issue, a malicious actor could under certain circumstances alter files. This has happened, possibly multiple times already.
* To mitigate it, I've run a script that changes the file permissions and owners to their bare minimum; system files can be read by the www-data user, but only written to by the root user for example.
* Unfortunately, this script is a bit too eager and also changed the owner / permissions of the forums subfolder, after we last updated WP. Boo.
* I fixed it with a tactical `chown -R www-data:www-data forums`
Apologies for the fucking up of shit. again.