You don't have the "groups" plugin enabled?

The _token/_ts fields should be generate automatically as hidden input fields in the group creation form.  If you view the source of the group creation page, do you see the __elgg_token and __elgg_ts fields?

The error message about ElggDiskFilestore::__construct() is unrelated.

PLai, thanks for your reply!

Yes, I have enabled the "groups" plugin, but no additional group-related plugins whatsoever.

When I view the source of the group creation page (/pg/groups/new), there is no "token" or "ts" or any similar field in the group creation form. The page source only contains the fields that are visible.

Edit: May this issue be related to the fact that I installed a second instance of Elgg (using a seperate document root, database, and data directory of course) on the same server? The second instance is configured to a second domain (I'm using apache virtual servers). Odd thing is, the fresh second instance of Elgg has the same issue when creating groups.
(Sorry if this may be confusing, I'm just trying to give as much info as possible).