Unable to change max filesize from Admin page

When I try to change the default upload filesize from 513MB to 10 GB it fails with the following error in the browser log:

POST https://<domain>/settings/admin/additional 405 (Method Not Allowed)
Navigated to https://<domain>/settings/admin/additional

No entries in the NC logfile seen.

the .htaccess file is owned by www-data:

-rw-r--r-- 1 www-data www-data 3654 Aug 24 16:52 .htaccess

I assume you are using the RC?

That should be fixed with https://github.com/nextcloud/server/pull/1022 and thus be in the final release