I'm not following the steps that you did. Could you write them down step by step?

I had written a step-by-step instruction somewhere on the community some time back. You could search for it.

OK, I solved it. Just set error_reporting to 1 and saw, that a plugin (qli_recaptcha) caused an php error (cannot redeclare class ...). Disabled the plugin and everything works fine. Sorry for my useless post.

I just searched all files for 'group_acl' but could only find it in mod/group/start.php So, it seems none of the group aware plugins like Blog/File/Page use it?

We created a new group aware plugin and that one seems to work just fine. Of course, users that create an object for a group should have an access right allowing the group members to see it... A little dirty (or wrong even?) maybe , but we set all access rights of the new objects to Public and use a separate metadata object that contains the access rights and users can set explicitly.  

I had another try with my plugin today. It seems to work allright now. So, I mainly do what I did before. Setting the access_id of my entity to the group (not the group_id, but the group ACL or how it is called) and using the group_id as container_id seems to do all the magic I need. Tom, why do you set the access_id to Public?

The plugins like blog do not need to use group_acl because it is used to in the creation of the access dropdown list. The only time you need to use it is when you are hard coding something to be group only.

What version of Elgg are you using?  $is_admin was deprecated by elgg_set_ignore_access(TRUE) in 1.6.

I'm using 1.6

Have you checked your PHP/Apache log to see if any errors are thrown?  Also, you can edit your .htaccess file to set "php_value display errors 1" to show any errors.