I have hit a dead-end with one of our Marketing Cloud Connect implementations. If anyone can throw in any suggestions, I'd be very grateful (are grasping at straws at this point).
After the Connector is all setup, even though we can see the reports and email templates, when we actually do a send, nothing occurs. Specifically, when we send to a report from Salesforce, it shows 'Processing' with a message of 'Data retrieval retried.' In Marketing Cloud, when sending to a report, the send never get's scheduled or appears in the Tracking. The API logs show a message of 'User Not Authenticated'.
Interestingly, this is a Multi-Org environment and we were able to connect another Salesforce Instance without issues (everything works).
We have narrowed it down to some form of an OAuth / Permissions issue, but we are ultimately hitting a roadblock.
We have followed every single step in the help documentation, including the troubleshooting guide (eg. re-change your password, make email opt-out visible to all).
We have set all the Salesforce System User / API user credentials to System Admins and relaxed all security restrictions. We have also manually whitelisted all the IPs.
We also manually went through each step that the Connector Setup Wizard goes through.
We also had a separate independent Marketing Cloud instance, manually mirrored the settings, hooked it up to our Salesforce Sandboxes and got it to work on that end (2 Salesforces connecting to 1 Marketing Cloud).
Long story short, we are open to any ideas out there.