What "worked"?  Removing ToList with that code alone means nothing will happen.  ToList forces the enumerable to be enumerated...

So it worked when you deferred the enumeration till later? What else was going on at that point?

Alternatively, try breaking the query up into steps and call .ToList() on each step until it breaks.

I've run into similar errors a number of times - it seems to happen whenever I send EF a query expression that's too complex for it to process.  Like Peter said, ToList() forces the enumeration (in this case, it forces EF to attempt to create and execute its query).  If you can, break the query up into smaller pieces that you pre-evaluate with ToList() and then use Linq-to-objects to handle the rest.  

Are you passing objects between 2 different contexts?

Do you have navigation properties on these objects? (E.g. ScheduledEvent.EmailSendRequests). You should use the Navigation properties on your entities in your query rather than constructing the join manually as you are doing.

Other than that I can't see anything wrong per se with the code that should generate that error.