FortiManager as SSL VPN bookmark results in SSL errors

Hi, 

we present internal web servers to external authenticated users with a FortiGate (5.4.6) and SSL VPN. While other web based tools work with this setup ironically the FortiManager (5.4.4) isn't cooperating. Chrome shows "ERR_EMPTY_RESPONSE" when opening the bookmark. 

The issue can be seen within the packet capture. After successful Client- and Server-Hello the connection is dropped by the Manager with "bad_record_mac" that means the decryption went wrong - ouch!

I guess this setup should work and I will need to open a case or am I missing something obvious?  

Cheers,

Dirk