Prevent users to edit the detailed description are a specific state


I Have a workflow for a Problem Report.

Phases are: CREATE=Initiated, ANALYSIS+WORK=Prepared->Assessed->Accepted->Implemented, FROZEN=Closed

-Transitions are set to roles: 

In most cases Assessor and TeamLeader role are use use by the same person.

In the states Initiated and Prepared the edit of the detailed description shall be allowed. This works fine.

From Assessed onwards the edit of the detailed description shall be limited to only a few users. This works only for Assessed and Implemented. 
In Accepted the edit is possible again, this should not be the case.

Attribute rules for PCMS_CHDOC_DETAIL_DESC were originally:

INITIATED->PREPARED=writeable/required for PREPARER

Later I added also 

ASSESSED->ACCEPTED=[nothing] for TeamLeader
ACCEPTED->IMPLEMENTED=[nothing] for TeamLeader
IMPLEMENTED->CLOSED=[nothing] for TeamLeader

without any change.

The privileges for Add/Edit Detailed Description is only enabled for "Object is in the inbox or user has a current rule". (when I deactivate this, edit is never possible)

For me it looks like the role TeamLeader can edit the detailed description in any state were it could also do a transition. 
How can I separate this?
The Problem Report is of course in the inbox of the Assessor/TeamLeader after the assessment and also for the implementation check. But the TeamLeader/Assessor shall only move the request to the next status and not edit the detailed description. In case of comments there is a separate attribute.

So my main question is: what all do I need to set up to prevent the edit of the detailed description after the second transition in the workflow?

Best Regards