Here's the answer from my customer:
So on number 1: No child domains are included, just one domain in one forest.
Number 2: Confirmed that the Sync user MSOL_###### is located in the "Users" container of the Forest root Domain.
And number 3: Confirmed that the Sync user MSOL_###### and the domain admin account are members of the FIM administrator group in both the local computer group and the domain group. Then I hit a problem, probably because we haven't been able to finish the configuration wizard for the Sync tool. When I look in the Synchronization Service Manager in the "Management Agents"-tab its empty. Attached a screenshot of it.
The SQL related problems exists on one server, the 2012 Essentials Server. The other server (2008 R2) did not have a SQL from the start.
- Well, no need to attach the screenshot since the Synchronization Service Manager on AZUREDIRSYNC is empty.
Thank you,
Jopi