Cannot enable mailbox (See the inner exception for details)

I am new to the Cloud Panel. Having this issue:

Trying to enable mailbox for a new user and getting “An error occurred while updating the entries. See the inner exception for details”.
I was able to create a new reseller, new company, then enabled Exchange for the company. It created ABP, address lists, accepted domain and security groups.

What am I missing here?

Thanks,
Daniel

My setup:
DC – Win 2012R2 – separate VM
Exchange 2013 CU9 – separate VM
Cloud Panel v3.1.0805 installed on a VM with SQL Express 2012R2