CRM-10865 Creating a case fails immediatly.

can you check if this happens on the demo server?

It does not happen on the demo server. I started debugging this, and I noticed that in the file CRM/Case/Form/Case.php around line 188, the variable $this->_activityTypeFile is empty. ( Activity type =13, which is "Open Case")

This issue is causing the class name "CRM_Case_Form_Activity_" to be passed as a parm to the CRM class loader function.

You'll need to debug this further. Looking at the code it seems like this function is returning FALSE instead of returning a file name for the Open Case activity:

CRM_Activity_BAO_Activity::getFileForActivityTypeId($this->_activityTypeId,
'Case'
)

I "thought" this might be caused by the expected option_value.name for that activity type being modified somehow. But that condition actually returns a very explicit fatal error:

The Open Case activity type is missing or disabled. Please have your site administrator check Administer > Option Lists > Activity Types for the CiviCase component.

So you'll need to add debug statements to figure out why getFileForActivityTypeId is failing in your install(s).

I found the issue, it was in a custom module that had overridden the CRM_Activity_BAO_Activity class. It was not an issue with CiviCRM core files.