Custom usernode view occasionally breaks

I've got a custom view which uses usernode fields, filters and exposed filters.
This view works fine most of the time. However, once a while when I's loaded, the table view itself isn't loaded (some of the exposed filters are) and I get this SQL error:

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 '= ',,') GROUP BY node.nid ORDER BY profile_initials_value ASC LIMIT 0, 50' at line 1 query: 

SELECT DISTINCT(node.nid), profile_initials.value AS profile_initials_value, profile_name_between.value AS profile_name_between_value, profile_surname.value AS profile_surname_value, profile_gender.value AS profile_gender_value, profile_birth_date.value AS profile_birth_date_value, usernode_users.name AS usernode_users_name, usernode_users.uid AS usernode_users_uid, usernode_users_roles.rid AS usernode_users_roles_rid, usernode_users_roles.uid AS usernode_users_roles_uid FROM node node INNER JOIN usernode usernode ON node.nid = usernode.nid LEFT JOIN users_roles usernode_users_roles ON usernode.uid = usernode_users_roles.uid LEFT JOIN users usernode_users ON usernode.uid = usernode_users.uid WHERE (node.type IN ('usernode')) AND (usernode_users_roles.rid IN ('2','9','5','6','4')) AND (.value = ',,') GROUP BY node.nid ORDER BY profile_initials_value ASC LIMIT 0, 50

As a temporary workaround, I manually enforced the views cache to be cleared on this particular view, but that doesn't prevent that sometimes you get a partially white screen. After a reload it works fine again.

Any ideas? I would be happy to follow any pointers or provide more information.