I created a filtered data extension from the SYNCHRONIZED Contact data extension from Salesforce. There were many fields on that DE, but only one EMAIL type field and I used the Contact ID as related to Subscriber on SubscriberKey. I tried a Guided Send to that filtered DE and it didn't work... Then I created a new DE populated with a query with only the 2 or 3 fields I need, and then the send worked...
Would you have any idea of why it failed with a filtered synchronized DE ? Is there a restriction in SFMC with synchronized DE or filtered synchronized DE?
I used the Contact ID as related to Subscriber on SubscriberKey
--> you shouldn't even have to do this