404 when calling toRepresentation()

Hi, what could be the reason that for some users (not different than the others in first glance) we get 404 error when calling userResource.toRepresentation() ?

It’s java application, using Tomcat adapter, Keycloak 9.0.0

When I get userResource and then call toRepresentation(), for some users I get 404:

Caused by: javax.ws.rs.NotFoundException: HTTP 404 Not Found
at org.jboss.resteasy.client.jaxrs.internal.ClientInvocation.handleErrorStatus(ClientInvocation.java:221) ~[resteasy-client-3.6.1.Final.jar:3.6.1.Final]
at org.jboss.resteasy.client.jaxrs.internal.ClientInvocation.extractResult(ClientInvocation.java:191) ~[resteasy-client-3.6.1.Final.jar:3.6.1.Final]
at org.jboss.resteasy.client.jaxrs.internal.proxy.extractors.BodyEntityExtractor.extractEntity(BodyEntityExtractor.java:60) ~[resteasy-client-3.6.1.Final.jar:3.6.1.Final]
at org.jboss.resteasy.client.jaxrs.internal.proxy.ClientInvoker.invokeSync(ClientInvoker.java:150) ~[resteasy-client-3.6.1.Final.jar:3.6.1.Final]
at org.jboss.resteasy.client.jaxrs.internal.proxy.ClientInvoker.invoke(ClientInvoker.java:112) ~[resteasy-client-3.6.1.Final.jar:3.6.1.Final]
at org.jboss.resteasy.client.jaxrs.internal.proxy.ClientProxy.invoke(ClientProxy.java:76) ~[resteasy-client-3.6.1.Final.jar:3.6.1.Final]
at com.sun.proxy.$Proxy146.toRepresentation(Unknown Source) ~[?:?]

User exists in Keycloak, all looks fine. I can’t really understand what can go wrong there.