Error 1267 - Newsletter Subscriber add-on

This is the error message I get when trying to email newsletter subscribers:

1267 Illegal mix of collations (latin1_general_ci,IMPLICIT) and (latin1_swedish_ci,IMPLICIT) for operation 'locate'
in:
[SELECT s.email_address as customers_email_address FROM znc_subscribers as s LEFT JOIN znc_configuration as q on LOCATE( s.email_address, q.configuration_value) >= 1 WHERE configuration_key = 'NEWSONLY_SUBSCRIPTION_TEST_GROUP' ]
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I've just added the Newsletter Subscriber add-on which allows 'customers' (not registered) to subscribe to the newsletter only.
Would someone point me in the right direction?