If you have set up a fresh Azure AD Connect installation (latest version as of today), go to add a new inbound sync rule, and you get the wonderful error message "Object reference not set to an instance of an object", where should I look for the root cause of the problem?
The client runs, I got no errors during installation, there is no info in event log. I have tried creating very simple rules, but I doubt it has anything to do with the actual rule - and I suspect something with permissions, but I have no idea permissions for what.
My account is in the ADSyncAdmins group and I have tried with a service account, that also is in that group - same error.
Looks like it was a bug in version 1.1.561.0 released July 23 2017. You just have to populate the "tag" attribute when setting up the new rule.
see the below release notes
https://docs.microsoft.com/en-us/azure/active-directory/connect/active-directory-aadconnect-version-history
"Fixed an issue where new synchronization rule cannot be created if the Tag attribute isn’t populated."