This is a bug. Did you use the latest version of Mendix, 2.4.6? If true, you can submit an incident with a little test project in MXDN.
However, with a little change of your XPath query, maybe the error does not occur. The nested constructions in the query are not necessary in this query, see query 1. When you search on an associated id, the id attribute and his entity can also be avoid, see the better query 2.
Query 1
//PipsAndSubbies.VATCode
[Audit.AuditEventAuditable/Audit.AuditEvent/
Audit.AuditEventAuditable/PipsAndSubbies.Supplier/id='281535106252805']
Query 2
//PipsAndSubbies.VATCode
[Audit.AuditEventAuditable/Audit.AuditEvent/
Audit.AuditEventAuditable='281535106252805']