Ward,
Without additional information on the exact setup of your version I was not able to replicate the issue exactly, but the widget is quite difficult to get correct.
It seems you are trying to search for data over an association. This will work when using the contains option, but not for the equals in my case below.
I have 2 entities:
1) Inspectie
2) Entity
They have a many to 1 relation Inspectie_Entity.
To search for the attribute (named Attribute) on the Entity entity the following config of the widget will work and search for the right objects:
In the List view entity I added the relation from my Inspectie entity to the associated entity.
If you change the search option to equals an error is thrown like below:
com.mendix.core.CoreRuntimeException: Exception occurred in action '{"xpath":"//MyFirstModule.Inspectie[MyFirstModule.Inspectie_Entity/MyFirstModule.Entity[equals(Attribute,'B')]]","amount":10,"offset":0,"returnsCount":true,"sort":[],"type":"RetrieveXPathSchemaRawAction"}', all database changes executed by this action were rolled back
And if you look further down it seems that the xpath can't be understood by the Mendix business server:
Caused by: java.lang.RuntimeException: Unknown predicate function: equals
I don't think this last part is the same error that you are getting, but your error is in the configuration of the widget.
Please compare the config with the one depicted above and make sure to have the latest version of the widget in your project.