Solved
Fortimail 5.3.5 and PAID Fortisandbox.Cloud!!
I've upgraded our Fortimail to 5.3.5. We have a valid Fortisandbox.Cloud Contract - now with 5.3.5 we can only send 100 Samples per day!! Level 1 Support says that this is per design! So be carefull when upgrading!!
On the Dashboard:
"Legacy FortiSandbox cloud service is used. Please activate FortiCloud as soon as possible!"