I have connected two switches over the Management Interface Gi0/0.
The issues takes affect only on one off them.
After the upgrade vom 3.6.(6)E to 3.6.(8)E it worked for two weeks without Problem.
But now I have allready the same issue again.
A other solution to get Access is to shutdown and up the port on the switch, where the
Management interface Gi0/0 is connected to.