As I told privately to support (without having any further answer), I think the problem happens when there are quotes in the name of the sender. This is an example of the search query that returns an error:
Mon Jul 15 14:56:53 2019 [db9b61ad-5286-ea1d-2700-5c8693c25481][FATAL] Error running count query for Contact List: Query Failed: SELECT count(*) c FROM contacts LEFT JOIN contacts_cstm ON contacts.id = contacts_cstm.id_c LEFT JOIN users jt0 ON contacts.modified_user_id=jt0.id AND jt0.deleted=0
AND jt0.deleted=0 LEFT JOIN users jt1 ON contacts.created_by=jt1.id AND jt1.deleted=0
AND jt1.deleted=0 LEFT JOIN users jt2 ON contacts.assigned_user_id=jt2.id AND jt2.deleted=0
AND jt2.deleted=0 LEFT JOIN contacts jt5 ON contacts.reports_to_id=jt5.id AND jt5.deleted=0
AND jt5.deleted=0 LEFT JOIN campaigns jt6 ON contacts.campaign_id=jt6.id AND jt6.deleted=0
AND jt6.deleted=0 LEFT JOIN vis1_visualizzatore1 jt11 ON contacts_cstm.vis1_visualizzatore1_id_c = jt11.id AND jt11.deleted=0 LEFT JOIN cad_cad jt12 ON contacts_cstm.cad_cad_id_c = jt12.id AND jt12.deleted=0 LEFT JOIN users jt13 ON contacts_cstm.user_id_c = jt13.id AND jt13.deleted=0 LEFT JOIN vis2_visualizzatore2 jt14 ON contacts_cstm.vis2_visualizzatore2_id_c = jt14.id AND jt14.deleted=0 LEFT JOIN cad_cad jt15 ON contacts_cstm.cad_cad_id1_c = jt15.id AND jt15.deleted=0 where (contacts.id in (SELECT eabr.bean_id FROM email_addr_bean_rel eabr JOIN email_addresses ea ON (ea.id = eabr.email_address_id) WHERE eabr.deleted=0 and **ea.email_address = ''Antonio**')) AND contacts.deleted=0: MySQL error 1064: You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'Antonio')) AND contacts.deleted=0' at line 11
The WHERE condition searches for ''Antonio' - that is A PART of the NAME of the sender - rather than for his email address. The wrong string has been taken from the message header, and it happens when there are quotes. As you can see, the starting quote is also taken, resulting in a syntax error.