A family of Microsoft relational database management systems designed for ease of use.
Have you perchance changed the name of the EndDate field in either table recently? This might be a problem with the notorious Name Autocorrect feature (nicknamed "Name Autocorrupt" by some of us). If your database properties are set to "Track Name Autocorrect", try turning that feature off, compacting the database, and see if the query now works.
You may also want to try copying the SQL of the query out to a file; delete the query; compact the database and recreate the query by pasting the SQL back in. It's possible that the compiled version of the query is out of step with the SQL, though that would be unusual.