Error processing 'file name': the field types of the join keys 'Join Key #1' and 'Join Key #2' do not match

The cause is most likely that the join keys are different field types or length.  Join keys need to be the same data type and length.  For example, a join key that is defined as a string=3 cannot be joined with a join key defined as string=12.  They need to be the exact same. 

Deleting one of the incoming datasets can also cause this error.  Deleting and recreating an input dataset can cause this error and require the join to be reset. You need to go back into your view and make sure all of the right files are still being pointed at.